SubmittingPatches: fix Asciidoc syntax in "GitHub CI" section
A superfluous ']' was added to the title of the GitHub CI section in
f003a91f5c (SubmittingPatches: replace discussion of Travis with GitHub
Actions, 2021-07-22). Remove it.
While at it, format the URL for a GitHub user's workflow runs of Git
between backticks, since if not Asciidoc formats only the first part,
"https://github.com/<Your", as a link, which is not very useful.
Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>