Error loading wikihow dataset

## Describe the bug

When attempting to load wikihow into a dataset with
```python
from datasets import load_dataset
dataset = load_dataset('wikihow', data_dir='./wikihow')
```
I get the message:
```
AttributeError: 'BuilderConfig' object has no attribute 'filename'
```
at the end of a [full stack trace](https://gist.github.com/odellus/602c3b2de52f541d353b1022f320ffc2).

## Steps to reproduce the bug

I have followed the instructions for creating a wikihow dataset. The [wikihow dataset site](https://huggingface.co/datasets/wikihow) says to use 
```python
from datasets import load_dataset
dataset = load_dataset('wikihow')
```
to load the dataset. I do so and I get the message
```
AssertionError: The dataset wikihow with config all requires manual data.
 Please follow the manual download instructions:   You need to manually download two wikihow files. An overview of which files to download can be seen at https://github.com/mahnazkoupaee/WikiHow-Dataset.
  You need to download the following two files manually:
    1) https://ucsb.app.box.com/s/ap23l8gafpezf4tq3wapr6u8241zz358 and save the file under <path/to/folder>/wikihowAll.csv
    2) https://ucsb.app.box.com/s/7yq601ijl1lzvlfu4rjdbbxforzd2oag and save the file under <path/to/folder>/wikihowSep.csv

  The <path/to/folder> can e.g. be "~/manual_wikihow_data".

  Wikihow can then be loaded using the following command `datasets.load_dataset("wikihow", data_dir="<path/to/folder>")`.
  .
 Manual data can be loaded with `datasets.load_dataset(wikihow, data_dir='<path/to/manual/data>')
```

So I create a directory `./wikihow` and download `wikihowAll.csv` and `wikihowSep.csv` into the new directory.

Then I run 
```python
from datasets import load_dataset
dataset = load_dataset('wikihow', data_dir='./wikihow')
```

that's when I get the [stack trace](https://gist.github.com/odellus/602c3b2de52f541d353b1022f320ffc2)

## Expected results
I expected it to load the downloaded files into a dataset.

## Actual results
```python
Using custom data configuration default-data_dir=.%2Fwikihow
Downloading and preparing dataset wikihow/default (download: Unknown size, generated: Unknown size, post-processed: Unknown size, total: Unknown size) to /home/azureuser/.cache/huggingface/datasets/wikihow/default-data_dir=.%2Fwikihow/0.0.0/58f42f8f0e4d459811a0f69aaab35870093830ccd58006769e7e1eb3e0e686c2...                                                    ---------------------------------------------------------------------------
AttributeError
Traceback (most recent call last)
<ipython-input-9-5e4d40142f30> in <module>
----> 1 dataset = load_dataset('wikihow',data_dir='./wikihow')
~/.local/lib/python3.6/site-packages/datasets/load.py in load_dataset(path, name, data_dir, data_files, split, cache_dir, features, download_config, download_mode, ignore_verifications, keep_in_memory, save_infos, script_version, use_auth_token, **config_kwargs)
745         try_from_hf_gcs=try_from_hf_gcs,
746         base_path=base_path,--> 
747         use_auth_token=use_auth_token,
748     )
749 
~/.local/lib/python3.6/site-packages/datasets/builder.py in download_and_prepare(self, download_config, download_mode, ignore_verifications, try_from_hf_gcs, dl_manager, base_path, use_auth_token, **download_and_prepare_kwargs)
577                     if not downloaded_from_gcs:
578                         self._download_and_prepare(                                                             -->
579                             dl_manager=dl_manager, verify_infos=verify_infos, **download_and_prepare_kwargs        
580                         )                                                                                          
581                     # Sync info
~/.local/lib/python3.6/site-packages/datasets/builder.py in _download_and_prepare(self, dl_manager, verify_infos, **prepare_split_kwargs)
632         split_dict = SplitDict(dataset_name=self.name)
633         split_generators_kwargs = self._make_split_generators_kwargs(prepare_split_kwargs)                      -->
634         split_generators = self._split_generators(dl_manager, **split_generators_kwargs)                            
635                                                                                                                     
636         # Checksums verification
~/.cache/huggingface/modules/datasets_modules/datasets/wikihow/58f42f8f0e4d459811a0f69aaab35870093830ccd58006769e7e1eb3e0e686c2/wikihow.py in _split_generators(self, dl_manager)
132
133         path_to_manual_file = os.path.join(
--> 134             os.path.abspath(os.path.expanduser(dl_manager.manual_dir)), self.config.filename                        
135         )                                                                                                           
136
AttributeError: 'BuilderConfig' object has no attribute 'filename'
```
## Versions
Paste the output of the following code:
```python
import datasets
import sys
import platform

print(f"""
- Datasets: {datasets.__version__}
- Python: {sys.version}
- Platform: {platform.platform()}
""")
```
```
- Datasets: 1.5.0
- Python: 3.6.9 (default, Jan 26 2021, 15:33:00)                                      [GCC 8.4.0]
- Platform: Linux-5.4.0-1046-azure-x86_64-with-Ubuntu-18.04-bionic
```