README.md links to rstlint.py in a dynamic head of `main` CPython
branch:

```
    https://github.com/python/cpython/blob/main/Doc/tools/rstlint.py
                                           ^^^^
```

However, after python/cpython#31097 is merged,
the link will stop working. To keep the link working, `main` needs to be
replaced with some concrete commit SHA.

`main` branch is failing, see https://dev.azure.com/python/cpython/_build/results?buildId=96616&view=logs&j=4db1505a-29e5-5cc0-240b-53a8a2681f75&t=a975920c-8356-5388-147c-613d5fab0171

Logs:

```
PATH=./venv/bin:$PATH sphinx-lint -i tools -i ./venv -i README.rst
No problems found.
PATH=./venv/bin:$PATH sphinx-lint ../Misc/NEWS.d/next/
[1] ../Misc/NEWS.d/next/Library/2022-02-09-00-53-23.[bpo-45863]().zqQXVv.rst:0: No newline at end of file (no-newline-at-end-of-file).
[1] ../Misc/NEWS.d/next/Build/2022-01-19-11-08-32.[bpo-46430]().k403m_.rst:0: No newline at end of file (no-newline-at-end-of-file).
2 problems with severity 1 found.
```

This PR fixes these two problems, so `main` is green again.

Related PR: #31097
CC @JulienPalard

Automerge-Triggered-By: GH:JulienPalard