update gstreamer-tools.md
For the use of matroskademux plugin, since gst-plugins-good was updated from 0.10 to 1.0, the PAD template has changed from video_%02d and audio_%02d to video_%u and audio_%u which generate video_0 and audio_0, respectively. (commit a19a4a69a) So the current example using old pad names(video_00/audio_00) causes the failure in linking pads. This patch changes the pad names into correct ones.