Fix doctest list
# What does this PR do?

We have `docs/source/en/model_doc/t5v1_1.mdx` in `documentation_tests.txt`, but the file is actually `docs/source/en/model_doc/t5v1.1.mdx`.

This cause doctest fail.

```
ERROR: file or directory not found: docs/source/en/model_doc/t5v1_1.mdx
```
(https://github.com/huggingface/transformers/runs/6104280137?check_suite_focus=true)
_The documentation is not available anymore as the PR was closed or merged._