doc: Fix GstUri doc typos
doc: Fix GstUri doc typos

* Use &perctn; instead of reserved character %
* NULL take two L

krichter722/gstreamer,centricular/gstreamer,krichter722/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,krichter722/gstreamer,centricular/gstreamer,jpxiong/gstreamer,jpxiong/gstreamer,shelsonjava/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,krichter722/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,shelsonjava/gstreamer,krichter722/gstreamer,shelsonjava/gstreamer,jpxiong/gstreamer,centricular/gstreamer,shelsonjava/gstreamer,centricular/gstreamer,shelsonjava/gstreamer,jpxiong/gstreamer