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>