Update README.md (#17641)

* Update README.md

* Update sample_publish_events_to_a_topic_using_sas_credential.py

Author: Rakshith Bhyravabhotla

sdk/eventgrid/azure-eventgrid/README.md

@@ -211,6 +211,9 @@ Use a dict-like representation to send to a topic with custom schema as shown be
 
 ```Python
 import os
+import uuid
+import datetime as dt
+from msrest.serialization import UTC
 from azure.core.credentials import AzureKeyCredential
 from azure.eventgrid import EventGridPublisherClient
 

sdk/eventgrid/azure-eventgrid/samples/sync_samples/sample_publish_events_to_a_topic_using_sas_credential.py

@@ -10,7 +10,8 @@
 USAGE:
     python sample_publish_events_to_a_topic_using_sas_credential.py
     Set the environment variables with your own values before running the sample:
-    1) EVENTGRID_SAS - The access key of your eventgrid account.
+    1) EVENTGRID_SAS - The shared access signature to use Event Grid. This is typically given to you
+    after creating it using the `generate_sas` method.
     2) EG_TOPIC_HOSTNAME - The topic hostname. Typically it exists in the format
     "https://<YOUR-TOPIC-NAME>.<REGION-NAME>.eventgrid.azure.net/api/events".
 """