change metadata (#280)

Author: shuangela

source/security/authentication.txt

@@ -18,7 +18,7 @@ Authentication Mechanisms
 
 .. meta::
    :keywords: community, security
-   :description: Learn how to authenticate to MongoDB using various authentication mechanisms compatible with different MongoDB Server editions.
+   :description: Learn how to authenticate to MongoDB using various authentication mechanisms in the PyMongo Driver compatible with different MongoDB Server editions.
 
 .. toctree::
    :caption: Authentication