res_rtp_asterisk: Correct default in sample configuration file.

With Asterisk 12 (commit 866d9681), 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