doc: do not bind osmo-mgw to random ip-address

The example config bind the MGW to a random ip-address, lets use the
loopback address here, this will suit cases where osmo-bts runs on the
same machine as the MGW (nitb). For all other cases were an external BTS
is used the ip-address still needs to be changed.

Change-Id: Iae52c671c48953ea6b52b18c5d77347343cde0df
1 file changed