Change register format example to match wording.

This change does not affect any other 1.6 branches as they have
already been updated for other changes, which uses the word 'domain'
as I have here.

(closes issue #15204)
Reported by: okrief

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.6.0@197088 65c4cc65-6c06-0410-ace0-fbb531ad65f3

configs/sip.conf.sample

@@ -400,7 +400,7 @@ srvlookup=yes                   ; Enable DNS SRV lookups on outbound calls
 ;----------------------------------------- OUTBOUND SIP REGISTRATIONS  ------------------------
 ; Asterisk can register as a SIP user agent to a SIP proxy (provider)
 ; Format for the register statement is:
-;       register => [transport://]user[:secret[:authuser]]@host[:port][/extension]
+;       register => [transport://]user[:secret[:authuser]]@domain[:port][/extension]
 ;
 ; 
 ;