Fix error in CTE association documentation

[JoeParker]

This PR addresses a mistake in the code examples for creating Common Table Expression associations.

The example given won't compile; updating the assignment operator to an equality check corrects the snippet and matches the similar example further on in the doc.

Pull Request Checklist

• [x] CONTRIBUTING: You have read https://github.com/groue/GRDB.swift/blob/master/CONTRIBUTING.md
• [x] BRANCH: This pull request is submitted against the development branch.
• [ ] DOCUMENTATION: Inline documentation has been updated.
• [x] DOCUMENTATION: README.md or another dedicated guide has been updated.
• [ ] TESTS: Changes are tested.
• [ ] TESTS: The make smokeTest terminal command runs without failure.

[groue]

Thank you @JoeParker,

This sounds like a welcome fix :) I'll review your PR in a few days when I'm back in front of a computer. Please hold on.

[groue]

I'm late! Thank you for your contribution, @JoeParker!