docs(aws-lambda): update execution role service principal (#25242)

There is a service principal mismatch between our `aws-lambda's Execution Role` example between [V1](https://docs.aws.amazon.com/cdk/api/v1/docs/aws-lambda-readme.html#execution-role) and [V2](https://docs.aws.amazon.com/cdk/api/v2/docs/aws-cdk-lib.aws_lambda-readme.html#execution-role) documentation.

This PR is updating the V1 doc's service principal from `sns.amazonaws.com` to `lambda.amazonaws.com`.

Closes #16427

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

@@ -105,7 +105,7 @@ it appropriate permissions:
 
 ```ts
 const myRole = new iam.Role(this, 'My Role', {
-  assumedBy: new iam.ServicePrincipal('sns.amazonaws.com'),
+  assumedBy: new iam.ServicePrincipal('lambda.amazonaws.com'),
 });
 
 const fn = new lambda.Function(this, 'MyFunction', {