docs(api-gateway): Fix IAM policy Effect enum case (#15417)

The Effect enum values were changed to be upper case in #2918 (b735d1c) but this usage in the docs was missed


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

@@ -509,7 +509,7 @@ iamUser.attachInlinePolicy(new iam.Policy(this, 'AllowBooks', {
   statements: [
     new iam.PolicyStatement({
       actions: [ 'execute-api:Invoke' ],
-      effect: iam.Effect.Allow,
+      effect: iam.Effect.ALLOW,
       resources: [ getBooks.methodArn ]
     })
   ]