DOCSP-42379: Update username/password placeholders

Author: mcmorisi

source/introduction/connect.txt

@@ -72,7 +72,7 @@ The following is an example of a connection URI that authenticates with MongoDB
 
 .. code-block:: text
 
-   mongodb://<username>:<password>@<hostname>:<port>/?authSource=<authenticationDb>&authMechanism=SCRAM-SHA-256
+   mongodb://<db_username>:<db_password>@<hostname>:<port>/?authSource=<authenticationDb>&authMechanism=SCRAM-SHA-256
 
 .. important:: Load Secrets