reflog: add missing single quote to error message

The error message can be seen by running
`git config gc.reflogexpire foo` and then `git reflog expire`.
Signed-off-by: NAlex Henrie <alexhenrie24@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/reflog.c

@@ -497,7 +497,7 @@ static int parse_expire_cfg_value(const char *var, const char *value, unsigned l
 	if (!value)
 		return config_error_nonbool(var);
 	if (parse_expiry_date(value, expire))
-		return error(_("%s' for '%s' is not a valid timestamp"),
+		return error(_("'%s' for '%s' is not a valid timestamp"),
 			     value, var);
 	return 0;
 }