To fix `nbformat` pre-commit instructions...

[jorgeorpinel]

The current instructions did not work from me and didn't seem to match the usage in the pre-commit docs. I believe that these changes will address the issue by

1. specifying that the given YAML contents go in a .pre-commit-config.yaml file;
2. clarifying that .pre-commit-hooks.yaml is a pre-commit plugin;
3. fixing the rev (there's no pre-commit rev so I used the only release tag I found); and
4. completing the config by specifying which hooks to run (from the plugin).

However, it still won't work as-is because .pre-commit-hooks.yaml is not located at the root of the repo (as indicated in https://pre-commit.com/#new-hooks). Am I missing something? E.g. is tools/tensorflow_docs/tools intended as some sort of subrepo?