Fix VTY: add missing doc for 'log gsmtap [HOSTNAME]'

Add missing doc string for the HOSTNAME arg.

Added in commit aa00f99be2e4cc64ede20d8c9548b83054696581, this breaks the VTY
tests for openbsc and hence breaks our jenkins build jobs.

Change-Id: I734b22c950242541322e902887bf779c14ba10fd
1 file changed