Update event-sources-system-topics.md

Author: spelluru

articles/event-grid/includes/event-sources-system-topics.md

@@ -1,6 +1,6 @@
 ---
  title: List of event sources in Azure Event Grid
- description: An include files that has the list of event sources supported by Azure Event Grid
+ description: An include file that has the list of event sources supported by Azure Event Grid.
  services: event-grid
  author: spelluru
  ms.service: event-grid