manual: add missing mention of MGCP in "Multiple instances"

Change-Id: I91d06921e4dca08428bf45fc1a3fd6e124599371
diff --git a/doc/manuals/chapters/running.adoc b/doc/manuals/chapters/running.adoc
index 03dade6..4bd3913 100644
--- a/doc/manuals/chapters/running.adoc
+++ b/doc/manuals/chapters/running.adoc
@@ -84,6 +84,7 @@
 
 - The SCCP/M3UA links are established by OsmoMSC contacting an STP.
 - The GSUP link is established by OsmoMSC contacting an HLR.
+- The MGCP link is established by OsmoMSC contacting an MGW.
 
 
 === Configure primary links