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

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