git-stash.txt: remove extra square bracket

git-stash.txt: remove extra square bracket

In 1ada5020b3 ("stash: use stash_push for no verb form", 2017-02-28),
when the pathspec argument was introduced in 'git stash', that was also
documented.  However I forgot to remove an extra square bracket after
the '--message' argument, even though the square bracket should have
been after the pathspec argument (where it was also added).

Remove the extra square bracket after the '--message' argument, to show
that the pathspec argument should be used with the 'push' verb.

While the pathspec argument can be used without the push verb, that's a
special case described later in the man page, and removing the first extra
square bracket instead of the second one makes the synopis easier to
understand.

Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>