docs: fix description of function parameter of shutil.register_archive_format

[brianschubert]

Both register_archive_format and register_unpack_format describe their function parameter as

function is the callable that will be used to unpack archives.

The former should instead read:

function is the callable that will be used to create archives.

The docstring of register_archive_format describes it correctly:

cpython/Lib/shutil.py

Lines 1160 to 1164 in c9380ae

	def register_archive_format(name, function, extra_args=None, description=''):
	"""Registers an archive format.
	name is the name of the format. function is the callable that will be
	used to create archives. If provided, extra_args is a sequence of

---

📚 Documentation preview 📚: https://cpython-previews--145087.org.readthedocs.build/

[github-actions]

This PR is stale because it has been open for 30 days with no activity.

[serhiy-storchaka]

LGTM. (How we did not see this?)

[miss-islington-app]

Thanks @brianschubert for the PR, and @serhiy-storchaka for merging it 🌮🎉.. I'm working now to backport this PR to: 3.13, 3.14, 3.15.
🐍🍒⛏🤖

[bedevere-app]

GH-150804 is a backport of this pull request to the 3.15 branch.

[bedevere-app]

GH-150805 is a backport of this pull request to the 3.14 branch.

[bedevere-app]

GH-150806 is a backport of this pull request to the 3.13 branch.