`<stream>` example should show multiple streams in a video file

[aelkiss]

The example in howtos/stream.md is a bit unusual (a WAV file embedded in an XML file), and it doesn't give a method to locate the WAV file within the XML file. An example of a video file containing multiple streams (as described in the text) might be more typical and a better example.

[jmlehton]

See the pull request