Add note to download_changed_screenshots.py about running test_data

Change-Id: I5cc7830626e8a94476261639db252e82af118924
diff --git a/tools/download_changed_screenshots.py b/tools/download_changed_screenshots.py
index 326b130..539da22 100755
--- a/tools/download_changed_screenshots.py
+++ b/tools/download_changed_screenshots.py
@@ -47,7 +47,8 @@
     output_path = path.join('test', 'data', 'ui-screenshots', screenshot_name)
     print(f'Downloading {url}')
     urllib.request.urlretrieve(url, output_path)
-  print('All done!')
+  print('Done. Now run:')
+  print('./tools/test_data upload')
 
 
 if __name__ == "__main__":