docs: change "Fixes:" tag example to match git fixes output

The "Fixes:" tag example has the commit title in double quotes, whereas the
suggested git fixes alias, a couple of lines below, also adds some outer
parenthesis.

Although there doesn't appear to be a consistent format for the "Fixes:" tag,
other than it should be a git commit sha followed by the commit title, the
information in the docs should at least be consistent. As the "Fixes:" tag was
inspired by the Linux kernel, which does have parenthesis, update the example to
match the git fixes output.

Signed-off-by: Frank Binns <frank.binns@imgtec.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6106>