TT#174407 Rename rtpproxy section to rtpengine

The rtpengine project used to be called rtpengine-ng, and before that
mediaproxy-ng, which was an implementation using the same protocol as
the rtpproxy project. It's about time we rename the config.yml section
so that this is less confusing.

Change-Id: I51fc5effc515ccca74b4d2093b9665edb20286f8

docs/ngcpcfg.txt

@@ -569,7 +569,7 @@ Add option '--diff' to display applied changes.
 Here you can find the most popular cases for YML configs modification:
 
   # change rtpengine max port number:
-  ngcpcfg set /etc/ngcp-config/config.yml "rtpproxy.maxport=44123"
+  ngcpcfg set /etc/ngcp-config/config.yml "rtpengine.maxport=44123"
   # disable asterisk service:
   ngcpcfg set /etc/ngcp-config/config.yml "asterisk.enable='no'"
   # add value with spaces