doc: fix forward reference about ghost pads
doc: fix forward reference about ghost pads

https://bugzilla.gnome.org/show_bug.cgi?id=711089

cablelabs/gstreamer,shelsonjava/gstreamer,ensonic/gstreamer,mparis/gstreamer,jpxiong/gstreamer,krichter722/gstreamer,Lachann/gstreamer,StreamUtils/gstreamer,krieger-od/gstreamer,cablelabs/gstreamer,Lachann/gstreamer,shelsonjava/gstreamer,jpxiong/gstreamer,ensonic/gstreamer,krichter722/gstreamer,krichter722/gstreamer,krieger-od/gstreamer,ensonic/gstreamer,jpxiong/gstreamer,jpakkane/gstreamer,cfoch/gstreamer,krichter722/gstreamer,jpxiong/gstreamer,krieger-od/gstreamer,mparis/gstreamer,cfoch/gstreamer,jpakkane/gstreamer,cablelabs/gstreamer,shelsonjava/gstreamer,surround-io/gstreamer,lubosz/gstreamer,StreamUtils/gstreamer,Lachann/gstreamer,mparis/gstreamer,shelsonjava/gstreamer,centricular/gstreamer,jpakkane/gstreamer,lovebug356/gstreamer,mparis/gstreamer,lubosz/gstreamer,cablelabs/gstreamer,lubosz/gstreamer,surround-io/gstreamer,mparis/gstreamer,lubosz/gstreamer,cfoch/gstreamer,jpakkane/gstreamer,centricular/gstreamer,lovebug356/gstreamer,krichter722/gstreamer,krieger-od/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,cfoch/gstreamer,centricular/gstreamer,cablelabs/gstreamer,ensonic/gstreamer,surround-io/gstreamer,StreamUtils/gstreamer,jpakkane/gstreamer,cfoch/gstreamer,jpxiong/gstreamer,lubosz/gstreamer,surround-io/gstreamer,ensonic/gstreamer,Lachann/gstreamer,centricular/gstreamer,krieger-od/gstreamer,surround-io/gstreamer,centricular/gstreamer,shelsonjava/gstreamer,Lachann/gstreamer,lovebug356/gstreamer,lovebug356/gstreamer,lovebug356/gstreamer