doc txt & -h consistency: balance unbalanced "[" and "]"

Fix a "-h" output syntax issue introduced when "--diagnose" was added
in aac0e8ffee (builtin/bugreport.c: create '--diagnose' option,
2022-08-12): We need to close the "[" we opened. The
corresponding *.txt change did not have the same issue.

The "help -h" output then had one "]" too many, which is an issue
introduced in b40845293b (help: correct the usage string in -h and
documentation, 2021-09-10).

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

builtin/bugreport.c

@@ -61,7 +61,7 @@ static void get_populated_hooks(struct strbuf *hook_info, int nongit)
 
 static const char * const bugreport_usage[] = {
 	N_("git bugreport [(-o|--output-directory) <file>] [(-s|--suffix) <format>]\n"
-	   "              [--diagnose[=<mode>]"),
+	   "              [--diagnose[=<mode>]]"),
 	NULL
 };