change the usage file sample (GoogleCloudPlatform#958)

since the file does not exist. Propose to use the same one as the tutorial: demomaker/gbikes_dinosaur.mp4

Author: merla18

video/cloud-client/shotchange/shotchange.py

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