update docs on required parameter for api_gateway_integration

* update docs on required parameter for api_gateway_integration

This parameter was required for lambda integration.

Otherwise,

` Error creating API Gateway Integration: BadRequestException: Enumeration value for HttpMethod must be non-empty`

* documentation: Including the AWS type on the api_gateway_integration docs

Author: stack72

website/source/docs/providers/aws/r/api_gateway_integration.html.markdown

@@ -52,7 +52,7 @@ The following arguments are supported:
   e.g. `arn:aws:apigateway:eu-west-1:lambda:path/2015-03-31/functions/arn:aws:lambda:eu-west-1:012345678901:function:my-func/invocations`
 * `credentials` - (Optional) The credentials required for the integration. For `AWS` integrations, 2 options are available. To specify an IAM Role for Amazon API Gateway to assume, use the role's ARN. To require that the caller's identity be passed through from the request, specify the string `arn:aws:iam::\*:user/\*`.
 * `integration_http_method` - (Optional) The integration HTTP method
-  (`GET`, `POST`, `PUT`, `DELETE`, `HEAD`, `OPTION`).
+  (`GET`, `POST`, `PUT`, `DELETE`, `HEAD`, `OPTION`). **Required** if `type` is `AWS` or `HTTP`.
   Not all methods are compatible with all `AWS` integrations.
   e.g. Lambda function [can only be invoked](https://github.com/awslabs/aws-apigateway-importer/issues/9#issuecomment-129651005) via `POST`.
 * `request_templates` - (Optional) A map of the integration's request templates.