STK Demo: Change the updated beat value to be published on the main thread

[markjeschke]

When I start the STK Demo, Xcode displays a warning, saying:

Publishing changes from background threads is not allowed; make sure to publish values from the main thread (via operators like receive(on:)) on model updates.

Adding DispatchQueue.main.async publishes the new values to the main thread.

[NickCulbertson]

Looks good. Thanks, Mark!