Changed registration WebLink prefix to 'reg'

Motivation:
Currently, section 3 'Registration' shows an example of a registration
response where the url for type 'urn:ietf:params:push:reg' is
'monitor/1G_GIPMorg_n-IrQvqZr6g'.
If I'm not mistaken this should be the same as the 'Location' header
returned, 'reg/1G_GIPMorg_n-IrQvqZr6g.

Modifications:
Changed 'monitor' to 'reg' in the WebLink