devicemonitor: fix typo in sample code in docs
devicemonitor: fix typo in sample code in docs

https://bugzilla.gnome.org/show_bug.cgi?id=735975

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