Remove extra curly brace from graphql query for Webhooks.get_webhook_id method

[JiaboHou]

Description

Removes an extra curly brace in the query for ShopifyAPI::Webhooks::Registry.get_webhook_id. When calling the unregister method by using ShopifyApp::WebhooksManager from the shopify_app gem I would get the following error:

ShopifyAPI::Errors::WebhookRegistrationError: Failed to fetch webhook from Shopify: Parse error on "}" (RCURLY) at [10, 1]
/Users/jiabo/src/github.com/Shopify/shopify-bids/lib/tasks/shopify_shops.rake:31:in `block in destroy_webhooks'

Which I believe is due to the extra closing bracket in the query.

How has this been tested?

I added unit tests for get_webhook_id, as well as tested this ]manually by specifying this branch as part of my Gemfile.

Checklist:

• My commit message follow the pattern described in here
• I have performed a self-review of my own code.
• I have added tests that prove my fix is effective or that my feature works.
• I have updated the project documentation. N/A
• I have added a changelog line.