docs: homogenize region tags (#194)

These samples' region tags slightly differ from identical snippets in different languages. The following is the remedy:

1. Merge this PR to fix GitHub source (keep old tag for now)
2. Fix doc code includes to represent the better region tag
3. Submit a second PR to clean-up redundant region tag wrapping

samples/samples/backup_sample.py

@@ -56,6 +56,7 @@ def create_backup(instance_id, database_id, backup_id):
 # [END spanner_create_backup]
 
 
+# [START spanner_restore_backup]
 # [START spanner_restore_database]
 def restore_database(instance_id, new_database_id, backup_id):
     """Restores a database from a backup."""
@@ -84,8 +85,10 @@ def restore_database(instance_id, new_database_id, backup_id):
 
 
 # [END spanner_restore_database]
+# [END spanner_restore_backup]
 
 
+# [START spanner_cancel_backup_create]
 # [START spanner_cancel_backup]
 def cancel_backup(instance_id, database_id, backup_id):
     spanner_client = spanner.Client()
@@ -116,6 +119,7 @@ def cancel_backup(instance_id, database_id, backup_id):
 
 
 # [END spanner_cancel_backup]
+# [END spanner_cancel_backup_create]
 
 
 # [START spanner_list_backup_operations]