docs: fix advanced-metadata code example in manual
docs: fix advanced-metadata code example in manual

https://bugzilla.gnome.org/show_bug.cgi?id=690751

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