provider/aws: Document the AWS_IAM authorizer type for api_gateway_method (#13214)

Fixes: #10497

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

@@ -39,7 +39,7 @@ The following arguments are supported:
 * `rest_api_id` - (Required) The ID of the associated REST API
 * `resource_id` - (Required) The API resource ID
 * `http_method` - (Required) The HTTP Method (`GET`, `POST`, `PUT`, `DELETE`, `HEAD`, `OPTION`, `ANY`)
-* `authorization` - (Required) The type of authorization used for the method (`NONE`, `CUSTOM`)
+* `authorization` - (Required) The type of authorization used for the method (`NONE`, `CUSTOM`, `AWS_IAM`)
 * `authorizer_id` - (Optional) The authorizer id to be used when the authorization is `CUSTOM`
 * `api_key_required` - (Optional) Specify if the method requires an API key
 * `request_models` - (Optional) A map of the API models used for the request's content type