DOCSP-11592: set mentioned driver version to 3.6 (#121)

Author: Chris Cho

source/index.txt

@@ -28,7 +28,7 @@ specify callbacks to access them when communicating with MongoDB.
 
 .. note::
 
-   These docs are for version 4.0 of the MongoDB Node.js driver. If
+   These docs are for version 3.6 of the MongoDB Node.js driver. If
    you are looking for an older version of the MongoDB Node.js driver
    docs, `see the legacy Node.js driver documentation
    <http://mongodb.github.io/node-mongodb-native/>`_.