Update doc issue on TableRequest.filter(keys:).

[GetToSet]

I'm working with GRDB.swift on my own app and after hitting the fatalError on TableRequest.filter(keys:), I found a minor issue on its documentation.

• Should be AND on the SQL statement.

Pull Request Checklist

• CONTRIBUTING: You have read https://github.com/groue/GRDB.swift/blob/master/CONTRIBUTING.md
• BRANCH: This pull request is submitted against the development branch.
• DOCUMENTATION: Inline documentation has been updated.
• 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 @GetToSet!

Please wait until I come back from vacations, in one week, until I merge your contribution 🙂

[groue]

Thank you!