To fix nbformat pre-commit instructions...

The current instructions did not work from me and didn't seem to match the usage in the [pre-commit docs](https://pre-commit.com/).

I believe that these changes (almost) address the issue by

a) specifying that the YAML contents go in a `.pre-commit-config.yaml` file`;
b) clarifying that `.pre-commit-hooks.yaml` is a pre-commit plugin; and
c) completing the config YAML by specifying which `hooks` to run (from the plugin)

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

tools/tensorflow_docs/tools/README.md

@@ -58,14 +58,16 @@ this, use a standard Git hook or use the
 [https://pre-commit.com/](https://pre-commit.com/) framework to create the hook
 for you.
 
-If you want to use pre-commit to handle the hook installation for you, include
-the [.pre-commit-hooks.yaml](./.pre-commit-hooks.yaml) file in your repo with
-the following contents:
+If you want to use pre-commit to handle the hook installation for you, create a
+`.pre-commit-config.yaml` file with the contents below to include the `nbformat`
+hook from the [.pre-commit-hooks.yaml](./.pre-commit-hooks.yaml) plugin.
 
 ```
 repos:
 - repo: https://github.com/tensorflow/docs
-  rev: pre-commit
+  rev: 2023.5.24.56664
+  hooks:
+  - id: nbformat
 ```
 
 Someone who clones that repo for development would then install the hook with: