Fix issue with c4 dataset creation

[nikita-savelyevv]

What does this PR do?

Condition needs to be updated because if enc.input_ids.shape[1] == seqlen then later the right random bound enc.input_ids.shape[1] - seqlen - 1 equals -1 which leads to an error.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

Who can review?

[nikita-savelyevv]

@echarlaix @IlyasMoutawwakil please take a look, thanks!