docs(rds): invalid master username (#5076)

## Commit Message
docs(rds): invalid master username (#5076)

## End Commit Message

Fixes the following error that occurs when `username` is set to `admin`:
```
 5/9 | 10:13:25 AM | CREATE_FAILED        | AWS::RDS::DBCluster                         | Database (DatabaseB269D8BB) MasterUsername admin cannot be used as it is a reserved word used by the engine (Service: AmazonRDS; Status Code: 400; Error Code: InvalidParameterValue; Request ID: 0ac76793-...)
```

packages/@aws-cdk/aws-rds/README.md

@@ -23,7 +23,7 @@ your instances will be launched privately or publicly:
 const cluster = new DatabaseCluster(this, 'Database', {
     engine: DatabaseClusterEngine.AURORA,
     masterUser: {
-        username: 'admin'
+        username: 'clusteradmin'
     },
     instanceProps: {
         instanceType: ec2.InstanceType.of(ec2.InstanceClass.BURSTABLE2, ec2.InstanceSize.SMALL),