Fix a bug in preact testing library usage example

[heatherKoo07]

Current example `expect(screen.textContent).toMatch("Current value: 6");`
failed with the following error message
    expect(received).toMatch(expected)
    Matcher error: received value must be a string
    Received has value: undefined
queries like getByText should be used to work with Async APIs
like waitFor or findBy

[developit]

LGTM, though @marvinhagemeister knows more than I do about testing-library.