Update mongo.rst

1. Use db.createUser() instead of db.addUser() to add users to MongoDB. Deprecated since version 2.6
2. NodeBB causes mongo error during setup procedure. Change the role from "userAdmin" to "readWrite".

docs/configuring/databases/mongo.rst

@@ -67,11 +67,11 @@ To add a user to access the `nodebb` database, type:
 
 .. code::
 
-    > db.addUser( { user: "nodebb",
+    > db.createUser( { user: "nodebb",
     ...       pwd: "<Enter in a secure password>",
-    ...       roles: [ "userAdmin" ] } )
+    ...       roles: [ "readWrite" ] } )
 
-**Note:** The role ``userAdmin`` gives all permissions to the user for that specific database.
+**Note:** The role ``readWrite`` provides read or write any collection within a specific database to user.
 
 Step 7: Configure MongoDB
 -------------------------