SubmittingPatches: fix Asciidoc syntax in "GitHub CI" section

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>