* Make checkout form submit event handler specific enough to match on…

[MaximilianoRicoTabo]

…ly checkout form and not others.

All Submissions:

• Have you followed the Contributing guideline?
• Does your code follow the WordPress' coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Make checkout submit event handler selector more specific to match only the checkout form and not others.

Resolves conflicts with gravity forms in checkout form for example

How to test the changes in this Pull Request:

1. Add a gravity form to checkout page.
2. Submit the gravity form
3. Try to checkout the level.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run tests with your changes locally?

Changelog entry

Enter a summary of all changes on this Pull Request. This will appear in the changelog if accepted.