Amplify Gen2 cli documentation refers to Backend construct that doesn't exist in aws-cdk-lib

**Describe the content issue**:

Section **Generate codegen for CDK app using a joint "AmplifyBackendStack" construct** in [CLI commands -reference](https://docs.amplify.aws/javascript/reference/cli-commands/) has following snippet:

```typescript
import { Construct } from 'constructs';
import { App, Backend } from 'aws-cdk-lib/aws-amplify';

export class MyAmplifyStack extends cdk.Stack {
constructor(scope: cdk.Construct, id: string, props?: cdk.StackProps) {
super(scope, id, props);

new Backend(this, "Backend", { /* ... */ });
}
}
```
But `Backend` construct doesn't exist in [`aws-cdk-lib/aws-amplify`](https://docs.aws.amazon.com/cdk/api/v2/docs/aws-cdk-lib.aws_amplify-readme.html) or [`@aws-cdk/aws-amplify-alpha`](https://docs.aws.amazon.com/cdk/api/v2/docs/aws-amplify-alpha-readme.html).

**URL page where content issue is**:

https://docs.amplify.aws/javascript/reference/cli-commands/