Fix en subset by modifying dataset_info with correct validation infos
- Related to: #2682 

We correct the values of `en` subset concerning the expected validation values (both `num_bytes` and `num_examples`.

Instead of having:

`{"name": "validation", "num_bytes": 828589180707, "num_examples": 364868892, "dataset_name": "c4"}`

We replace with correct values:

`{"name": "validation", "num_bytes": 825767266, "num_examples": 364608, "dataset_name": "c4"}`

There are still issues with validation with other subsets, but I can't download all the files, unzip to check for the correct number of bytes. (If you have a fast way to obtain those values for other subsets, I can do this in this PR ... otherwise I can't spend those resources)
