Documentation: update git configuration for Link: tag

The latest version of git (2.41.0) changed the spelling
of Message-Id to Message-ID. Adjust the perl script here
to accept both spellings.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Tested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20230619115533.981f6abaca01.I1960c39b1d61e8514afcef4806a450a209133187@changeid