add period

Author: rachelsaunders

firestore-shorten-urls/POSTINSTALL.md

@@ -13,7 +13,7 @@ You can test out this extension right away:
 ### Using the extension
 
 This extension listens to the Cloud Firestore collection `${param:COLLECTION_PATH}`. If you add a URL to the field `${param:URL_FIELD_NAME}` in any document within that collection, this extension:
-- Shortens the URL
+- Shortens the URL.
 - Saves the shortened URL in the `${param:SHORT_URL_FIELD_NAME}` field of the same document like so:
 
 ```