Fix wrong use of EVT_COMMAND in the example in wxThread documentation.

As the event being defined is a wxThreadEvent, EVT_THREAD must be used.

Also rename wxEVT_COMMAND_MYTHREAD_UPDATE to myEVT_THREAD_UPDATE to avoid
using "wxEVT_" prefix reserved by wx itself.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74159 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775