update docs

Author: ArturSantosTD

receiver/mongodbatlasreceiver/README.md

@@ -34,6 +34,7 @@ In order to collect access logs, the requesting API key needs the appropriate pe
 
 MongoDB Atlas [Documentation](https://www.mongodb.com/docs/atlas/reference/api/logs/#logs) recommends a polling interval of 5 minutes.
 
+- `base_url` (optional) set to https://cloud.mongodbgov.com to connecto to Atlas Gov
 - `public_key` (required for metrics, logs, or alerts in `poll` mode)
 - `private_key` (required for metrics, logs, or alerts in `poll` mode)
 - `granularity` (default `PT1M` - See [MongoDB Atlas Documentation](https://docs.atlas.mongodb.com/reference/api/process-measurements/))