Add missing period to index.yml summary property (#16734)

Author: scottaddie

docs/whats-new/index.yml

@@ -1,7 +1,7 @@
 ### YamlMime:Landing
 
 title: .NET documentation - what's new?
-summary: "Welcome to what's new in .NET docs. Use this page to quickly find the latest changes"
+summary: "Welcome to what's new in .NET docs. Use this page to quickly find the latest changes."
 
 metadata:
   title: .NET documentation what's new?