[docs] Fix decode examples and expected output in fast_tokenizers
Author: samyuktahegde
Pull request: yes

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# What does this PR do?

While working through the Fast Tokenizers documentation to understand encode()/decode(), I observed a few inconsistencies. This PR updates the examples to match the observed behavior and keeps the documentation consistent across the page.

### Documentation Changes 
**Encode and decode** section:  
- Replaces `tokenizer.decode(outputs["input_ids"])` with `tokenizer.decode(input_ids)`
as tokenizer.encode() returns just a list of token IDs rather than an object.
- Updates the expected output of `tokenizer.decode()` to display that it returns a string (instead of a list).

**Special tokens** section:  
- Applies the same corrections to the repeated `decode()` example by replacing `outputs["input_ids"]` with `input_ids` and updating the expected output to a string.


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Fixes # (issue)

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