test/zdtm.sh: fix an error message

It's either "are different" or "differ".
Let's use the shorter form.
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Acked-by: Andrew Vagin <avagin@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

test/zdtm.sh

@@ -340,7 +340,7 @@ diff_fds()
 {
 	test -n "$PIDNS" && return 0
 	if ! diff -up $1 $2; then
-		echo ERROR: Sets of descriptors are differ:
+		echo ERROR: Sets of descriptors differ:
 		echo $1
 		echo $2
 		return 1