am: put spaces around pipe in usage string

This makes the style a little more consistent with other usage strings,
and will resolve a warning at
https://www.softcatala.org/recursos/quality/git.html

Signed-off-by: Alex Henrie <alexhenrie24@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/am.c

@@ -2222,7 +2222,7 @@ int cmd_am(int argc, const char **argv, const char *prefix)
 	int in_progress;
 
 	const char * const usage[] = {
-		N_("git am [<options>] [(<mbox>|<Maildir>)...]"),
+		N_("git am [<options>] [(<mbox> | <Maildir>)...]"),
 		N_("git am [<options>] (--continue | --skip | --abort)"),
 		NULL
 	};