configs/basic-pbx: Modified main IVR to play new Allison prompt.

The main IVR was playing demo-congrats. I've switched it over to the
basic-pbx-ivr-main file that we added in core sounds 1.4.27. This prompt
has Allison prompting the user with the actual IVR menu.

ASTERISK-24892 #close

Change-Id: Ifb749616ff8e156a1031ddaddfcc9244767a095d

configs/basic-pbx/extensions.conf

@@ -159,9 +159,9 @@ exten = _25655511XX,1,Verbose(1, "External caller dialed inbound to DID ${EXTEN}
 [Main-IVR]
 exten = 2565551100,1,Verbose(1, "New caller, ${CALLERID(num)} dialed into the IVR.")
  same = n,Answer()
- same = n(start),Background(demo-congrats)
+ same = n(start),Background(basic-pbx-ivr-main)
  same = n,WaitExten(10)
- same = n,Background(demo-congrats)
+ same = n,Background(basic-pbx-ivr-main)
  same = n,Hangup()
 
 exten = 0,1,Verbose(1, "Caller ${CALLERID(num)} dialed the operator.")