Pass multi_use=True to spanner read-only transaction (GoogleCloudPlatform#1063)

Change-Id: Ied9d9f519edd572d79dc95d2812c1b98f5a92794

Author: Jon Wayne Parrott

spanner/cloud-client/snippets.py

@@ -378,7 +378,7 @@ def read_only_transaction(instance_id, database_id):
     instance = spanner_client.instance(instance_id)
     database = instance.database(database_id)
 
-    with database.snapshot() as snapshot:
+    with database.snapshot(mutli_use=True) as snapshot:
         # Read using SQL.
         results = snapshot.execute_sql(
             'SELECT SingerId, AlbumId, AlbumTitle FROM Albums')