webrtc_sendrecv.py: Link pads instead of elements

This was not a problem here because even if we end up accidentally linking to the wrong pad, things will work out eventually as long as one pad-added is emitted for each pad that is added.

But it will be a huge problem if someone copies this code and changes something that requires different handling for different sorts of pads. The resultant code will be racy. Let's not do this, it's a bad example.