Fix typos and missing videoconvert element to demonstrate
the alphacombine element.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1494>
*
* ## Example launch line
* |[
- * gst-launch-1.0 -v videotestsrc ! .c videotestsrc pattern=ball ! .c
- * alphacombine name=c ! compositor ! autovideosink
+ * gst-launch-1.0 -v videotestsrc ! c. videotestsrc pattern=ball ! c.
+ * alphacombine name=c ! compositor ! videoconvert ! autovideosink
* ]| This pipeline uses luma of a ball test pattern as alpha, combined with
* default test pattern and renders the resulting moving ball on a checker
* board.