[mutter] tests: Fix usage message for "set_parent"

[Jonas Ådahl <jadahl src gnome org>]

commit 22aea3dc67e573d979e0c9a91325194d5e3c5aff
Author: Olivier Fourdan <ofourdan redhat com>
Date:   Wed Jul 18 14:25:30 2018 +0200

    tests: Fix usage message for "set_parent"
    
    "set_parent" usage message was referring to "menu" which is not a valid
    command.

 src/tests/test-client.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/tests/test-client.c b/src/tests/test-client.c
index 1a29c15af..214f78181 100644
--- a/src/tests/test-client.c
+++ b/src/tests/test-client.c
@@ -144,7 +144,7 @@ process_line (const char *line)
     {
       if (argc != 3)
         {
-          g_print ("usage: menu <window-id> <parent-id>");
+          g_print ("usage: set_parent <window-id> <parent-id>");
           goto out;
         }