grep: correct help string for --exclude-standard

The current help string is about --no-exclude-standard. But "git grep -h"
would show --exclude-standard instead. Flip the string. See 0a93fb8
(grep: teach --untracked and --exclude-standard options - 2011-09-27)
for more info about these options.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/grep.c

@@ -643,7 +643,7 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 		OPT_BOOL(0, "untracked", &untracked,
 			N_("search in both tracked and untracked files")),
 		OPT_SET_INT(0, "exclude-standard", &opt_exclude,
-			    N_("search also in ignored files"), 1),
+			    N_("ignore files specified via '.gitignore'"), 1),
 		OPT_GROUP(""),
 		OPT_BOOL('v', "invert-match", &opt.invert,
 			N_("show non-matching lines")),