res_rtp_asterisk: Correct default in sample configuration file.

With Asterisk 12 (commit 866d968), the default of "icesupport" changed to
- "yes" in the module "res_rtp_asterisk" and
- "no" in the module "chan_sip".
The latter was reflected in the sample configuration file for "sip.conf". The
former did not make it into "rtp.conf.sample".

ASTERISK-20643

Change-Id: I2a2e0a900455d0767a99ea576e30adc6d7608a36

configs/samples/rtp.conf.sample

@@ -33,8 +33,8 @@ rtpend=20000
 ; connected. This option is set to 4 by default.
 ; probation=8
 ;
-; Whether to enable or disable ICE support. This option is disabled by default.
-; icesupport=true
+; Whether to enable or disable ICE support. This option is enabled by default.
+; icesupport=false
 ;
 ; Hostname or address for the STUN server used when determining the external
 ; IP address and port an RTP session can be reached at. The port number is