Update capture-warnings.rst: Fix PEP number in link

[SamB]

This was linking to a completely unrelated PEP, presumably due to a typo.

Sorry there's no changelog, filing from my phone.

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Thanks for submitting a PR, your contribution is really appreciated!

Here is a quick checklist that should be present in PRs.

• Include documentation when adding new features. (Not a feature.)
• Include new tests or update existing tests when applicable. (Testing this would be probabilistic at best.)
• Allow maintainers to push and squash when merging my commits. Please uncheck this if you prefer to squash the commits yourself.

If this change fixes an issue, please:

• Add text like closes #XYZW to the PR description and/or commits (where XYZW is the issue number). See the github docs for more information. (I don't know of an issue for this.)

Unless your change is trivial or a small documentation fix (e.g., a typo or reword of a small section) (it is) please:

- [ ] Create a new changelog file in the `changelog` folder, with a name like `..rst`. See [changelog/README.rst](https://github.com/pytest-dev/pytest/blob/main/changelog/README.rst) for details.

Write sentences in the past or present tense, examples:

• Improved verbose diff output with sequences.
• Terminal summary statistics now use multiple colors.

Also make sure to end the sentence with a ..

• Add yourself to AUTHORS in alphabetical order.

[nicoddemus]

Thanks!