correct incorrect dimension comment in Longformer model
This PR fixes a comment that incorrectly states the dimensions of a certain tensor in the `Longformer` model, confusing any reader trying to understand the code. The comment for the corresponding `TFLongformerX` is correct. 

- [x ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).

Tagging @patrickvonplaten  (Longformer)

