dataflow/flex-templates: adding reference to bug (GoogleCloudPlatform#4611)

Adding reference to a bug to keep track of to re-enable a test.

Author: David Cavazos

dataflow/flex-templates/streaming_beam/streaming_beam_test.py

@@ -127,7 +127,7 @@ def test_dataflow_flex_templates_pubsub_to_bigquery(dataset, topic_path,
 
 # TODO:Testcase using Teststream currently does not work as intended.
 # The first write to BigQuery fails. Have filed a bug. The test case
-# to be changed once the bug gets fixed.
+# to be changed once the bug gets fixed.  b/152446921
 '''
 @mock.patch("apache_beam.Pipeline", TestPipeline)
 @mock.patch(