transport-helper: fix typo in error message when --signed is not supported

Signed-off-by: NMike Hommey <mh@glandium.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

transport-helper.c

@@ -865,7 +865,7 @@ static int push_refs_with_export(struct transport *transport,
 			die("helper %s does not support dry-run", data->name);
 	} else if (flags & TRANSPORT_PUSH_CERT) {
 		if (set_helper_option(transport, TRANS_OPT_PUSH_CERT, "true") != 0)
-			die("helper %s does not support dry-run", data->name);
+			die("helper %s does not support --signed", data->name);
 	}
 
 	if (flags & TRANSPORT_PUSH_FORCE) {