docs(lambda): add missing JSDoc Markdown code block (#29348)

### Reason for this change

There is a missing Markdown code block  in the [`EventSourceMapping` documentation](https://docs.aws.amazon.com/cdk/api/v2/docs/aws-cdk-lib.aws_lambda.EventSourceMapping.html):

![image](https://github.com/aws/aws-cdk/assets/2505696/cb50ded7-a4b0-43f9-ace7-736d805b23d0)


### Description of changes

Adds missing Markdown code block tags

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/aws-cdk-lib/aws-lambda/lib/event-source-mapping.ts

@@ -292,8 +292,15 @@ export interface IEventSourceMapping extends cdk.IResource {
  * Usually, you won't need to define the mapping yourself. This will usually be done by
  * event sources. For example, to add an SQS event source to a function:
  *
- *    import { SqsEventSource } from 'aws-cdk-lib/aws-lambda-event-sources';
- *    lambda.addEventSource(new SqsEventSource(sqs));
+ * ```ts
+ * import * as sqs from 'aws-cdk-lib/aws-sqs';
+ * import * as eventsources from 'aws-cdk-lib/aws-lambda-event-sources';
+ *
+ * declare const handler: lambda.Function;
+ * declare const queue: sqs.Queue;
+ *
+ * handler.addEventSource(new eventsources.SqsEventSource(queue));
+ * ```
  *
  * The `SqsEventSource` class will automatically create the mapping, and will also
  * modify the Lambda's execution role so it can consume messages from the queue.