interleave: Fix the example by setting channel-masks in the sink pads

interleave: Fix the example by setting channel-masks in the sink pads

The current example does not work, it fails with:

ERROR: from element /GstPipeline:pipeline0/GstDecodeBin:decodebin0/GstWavParse:wavparse0: Internal data flow error.
gstwavparse.c(2178): gst_wavparse_loop (): /GstPipeline:pipeline0/GstDecodeBin:decodebin0/GstWavParse:wavparse0:
streaming task paused, reason not-negotiated (-4)

This is because negotiation with wavenc gets messed up by the missing
channel positions configuration.

The proper way to define the channel layout when using the interleave
element in code would be to set the channel-positions property, but
gst-launch-1.0 does not know how to deal with arrays; so the example
pipeline works around the issue by setting the channel-masks in the sink
pads.

Also fix a repetition in the deinterleave example description

https://bugzilla.gnome.org/show_bug.cgi?id=735673