Changed read_write minimum amount to 300,000 as per bug [(googleapis#818

)](GoogleCloudPlatform/python-docs-samples#818)

samples/samples/snippets.py

@@ -320,7 +320,7 @@ def update_albums(transaction):
 
         transfer_amount = 200000
 
-        if second_album_budget < transfer_amount:
+        if second_album_budget < 300000:
             # Raising an exception will automatically roll back the
             # transaction.
             raise ValueError(