Update README documentation.

Update the README documentation to correctly describe which CLI command you should
use when attempting to get help from the CLI.

(closes issue #16064)
Reported by: thedavidfactor
Patches:
      readme.patch uploaded by thedavidfactor (license 903)

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@225515 65c4cc65-6c06-0410-ace0-fbb531ad65f3

README

@@ -146,8 +146,8 @@ like this:
 
 *CLI>
 
-  You can type "help" at any time to get help with the system.  For help
-with a specific command, type "help <command>".  To start the PBX using
+  You can type "core show help" at any time to get help with the system.  For help
+with a specific command, type "core show help <command>".  To start the PBX using
 your sound card, you can type "console dial" to dial the PBX.  Then you can use
 "console answer", "console hangup", and "console dial" to simulate the actions
 of a telephone.  Remember that if you don't have a full duplex sound card