docs: replace formId with id in examples

[mosesintech]

What

This PR replaces formId with id in the example submission mutations.

Why

Attempting to submit a mutation using formId gives this error:
There was an error submitting the form: [GraphQL] Field SubmitGfFormInput.id of required type ID! was not provided. [GraphQL] Field "formId" is not defined by type SubmitGfFormInput.

How

No implementation, just updating the docs to reflect the correct mutation.

Testing Instructions

Additional Info

Checklist:

• This PR is tested to the best of my abilities.
• This PR follows the WordPress Coding Standards.
• This PR has proper inline documentation.
• This PR has unit tests to verify the code works as intended.
• The changes in this PR have been noted in CHANGELOG.md

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