Correct improper comment

"Delete server before trying to create server" looks confusing,
the author may would have wanted to say "Delete server before
trying to create image".

Change-Id: Ia0f8ca29d6042d022cf4f6edf60f149f4a5f9154
diff --git a/tempest/api/compute/images/test_images_negative.py b/tempest/api/compute/images/test_images_negative.py
index c48367f..549262e 100644
--- a/tempest/api/compute/images/test_images_negative.py
+++ b/tempest/api/compute/images/test_images_negative.py
@@ -46,7 +46,7 @@
         # An image should not be created if the server instance is removed
         server = self.create_test_server(wait_until='ACTIVE')
 
-        # Delete server before trying to create server
+        # Delete server before trying to create image
         self.servers_client.delete_server(server['id'])
         waiters.wait_for_server_termination(self.servers_client, server['id'])
         # Create a new image after server is deleted