feat: add missing return to README example

[g1stavo]

The compiler return a missing return error in this README code example:

	// Pass in test case. This is the component that makes the external HTTP call
	var test = func() (err error) {
		u := fmt.Sprintf("http://localhost:%d/foobar", pact.Server.Port)
		req, err := http.NewRequest("GET", u, strings.NewReader(`{"name":"billy"}`))
		if err != nil {
			return err
		}

		// NOTE: by default, request bodies are expected to be sent with a Content-Type
		// of application/json. If you don't explicitly set the content-type, you
		// will get a mismatch during Verification.
		req.Header.Set("Content-Type", "application/json")
		req.Header.Set("Authorization", "Bearer 1234")

		if _, err = http.DefaultClient.Do(req); err != nil {
			return err
		}
	}

Just added a return after the last if. Also omitted return values, as the function has a named return err error.