Update example README for macOS template
Update example README for macOS template

Now that `flutter create` supports macOS, update the example
README accordingly.

Also removes the instructions for modifying the example in a fork;
having two options is needlessly complicated, and copying the
folders works in a wider variety of cases (e.g., also supporting mobile).
google/flutter-desktop-embedding,google/flutter-desktop-embedding,google/flutter-desktop-embedding,google/flutter-desktop-embedding,google/flutter-desktop-embedding