overview: fix graph "OsmoMGW used with OsmoMSC"

The graph "OsmoMGW used with OsmoMSC" shows a BSC entity that accepts
AoIP and RTP, however osmo-bsc has no RTP proxy functionality. It makes
probably more sense to remove the BTS from the graph and call "BSC" "2G
BSS"

Change-Id: I6c7d26b6e45d0f953b82bca3ae7b2f26600982b2
diff --git a/doc/manuals/chapters/overview.adoc b/doc/manuals/chapters/overview.adoc
index bbca251..5179763 100644
--- a/doc/manuals/chapters/overview.adoc
+++ b/doc/manuals/chapters/overview.adoc
@@ -36,10 +36,9 @@
 ----
 digraph G {
     rankdir = LR;
-    BTS -> BSC [label="Abis"];
-    BSC -> OsmoMSC [label="3GPP AoIP"];
+    "2G BSS" -> OsmoMSC [label="3GPP AoIP"];
     OsmoMSC -> OsmoMGW [label="MGCP"];
-    BSC -> OsmoMGW [label="RTP",dir=both];
+    "2G BSS" -> OsmoMGW [label="RTP",dir=both];
     OsmoMSC -> OsmoSIP [label="MNCC"];
     OsmoSIP -> PBX [label="SIP Trunk"];
     OsmoMGW -> PBX [label="RTP",dir=both];