Update js_usage.markdown to use onRuntimeInitialized
Update js_usage.markdown to use onRuntimeInitialized

The Emscripten library is not guaranteed to be fully loaded during the
script element's onload event. Module.onRuntimeInitialized seems to be
more reliable.

opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv