reflog: remove 'show' from 'expire's usage string

Most of 'expire's options are not recognized by the 'show' subcommand,
hence it errors out.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: szeder

builtin-reflog.c

@@ -13,7 +13,7 @@
  */
 
 static const char reflog_expire_usage[] =
-"git reflog (show|expire) [--verbose] [--dry-run] [--stale-fix] [--expire=<time>] [--expire-unreachable=<time>] [--all] <refs>...";
+"git reflog expire [--verbose] [--dry-run] [--stale-fix] [--expire=<time>] [--expire-unreachable=<time>] [--all] <refs>...";
 static const char reflog_delete_usage[] =
 "git reflog delete [--verbose] [--dry-run] [--rewrite] [--updateref] <refs>...";