help doc: add missing "]" to "[-a|--all]"

Add a missing "]" to documentation added in 63eae83 (help: add "-a
--verbose" to list all commands with synopsis, 2018-05-20). This made
it seem as though "--[no-]verbose" can only be provided with "--all",
not "-a". The corresponding usage information in the C
code ("builtin_help_usage") does not have the same problem.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: avar

Documentation/git-help.txt

@@ -8,7 +8,7 @@ git-help - Display help information about Git
 SYNOPSIS
 --------
 [verse]
-'git help' [-a|--all [--[no-]verbose]]
+'git help' [-a|--all] [--[no-]verbose]
 	   [[-i|--info] [-m|--man] [-w|--web]] [<command>|<guide>]
 'git help' [-g|--guides]
 'git help' [-c|--config]