README: Update doc on how to update generated files
README: Update doc on how to update generated files

Fix the command, and also document how it can be done with gst-build

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer-sharp/-/merge_requests/25>

GStreamer/gstreamer-sharp,GStreamer/gstreamer-sharp,gstreamer-sharp/gstreamer-sharp,GStreamer/gstreamer-sharp,gstreamer-sharp/gstreamer-sharp,gstreamer-sharp/gstreamer-sharp