Update mongodbatlas.mdx (#21704)

Fix the error in the example for client cert credentials.

Co-authored-by: Yoko Hyakuna <yoko@hashicorp.com>

website/content/docs/secrets/databases/mongodbatlas.mdx

@@ -87,7 +87,7 @@ the proper permissions, it can generate credentials.
     ```shell-session
     $ vault write database/roles/my-dynamic-certificate-role \
       db_name=my-mongodbatlas-database \
-      creation_statements='{"database_name": "$external", "x509Type": "CUSTOMER", "roles": [{"databaseName":"$external","roleName":"readWrite"}]}' \
+      creation_statements='{"database_name": "$external", "x509Type": "CUSTOMER", "roles": [{"databaseName":"<db_name>","roleName":"readWrite"}]}' \
       default_ttl="1h" \
       max_ttl="24h" \
       credential_type="client_certificate" \