overview: fix graph "OsmoMGW used with OsmoBSC"

The graph "OsmoMGW used with OsmoBSC" shows an RTP and 3GPP AoIP going
to OsmoMSC, this is technically wrong since osmoMSC has no RTP proxy
functionality whatsoever. Since the graph is not so much about OsmoMSC
it makes more sense to label OsmoMSC with "core-network", then it makes
sense and correctly highlights the role of osmo-mgw on the BSS side.

Change-Id: If3550a101e1e3cc2a25e21dcfe5474134015a6a7
1 file changed