docs(apigateway): set integrationHttpMethod for vpc link integration (#28299)

As written in #6404, a VPC link integration fails to deploy unless we set `integrationHttpMethod` prop explicitly.

In this PR we fix the sample code accordingly.
 
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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: tmokmss

packages/aws-cdk-lib/aws-apigateway/README.md

@@ -1453,6 +1453,7 @@ const link = new apigateway.VpcLink(this, 'link', {
 
 const integration = new apigateway.Integration({
   type: apigateway.IntegrationType.HTTP_PROXY,
+  integrationHttpMethod: 'ANY',
   options: {
     connectionType: apigateway.ConnectionType.VPC_LINK,
     vpcLink: link,