change the usage file as it does not exist (GoogleCloudPlatform#959)

the gcs file  in the Usage sample does not exist. Replaced it with demomaker/google_gmail.mp4. Hopefully this will also update the readme

Author: merla18

video/cloud-client/faces/faces.py

@@ -22,7 +22,7 @@
 
 Usage Example:
 
-    python faces.py gs://cloudmleap/video/googlework.mp4
+    python faces.py gs://demomaker/google_gmail.mp4
 
 """