README.md: Fix sample code

[mingchen]

Fix sample code syntax in README.md

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 very much, @mingchen!