Update extensions.lua.sample with naming conflict guidance.

Update extensions.lua.sample with naming conflict guidance.

The sample extensions.lua was causing pbx_lua to fail to load when parsing
'app.goto("default", "s", 1)' because in Lua 5.2, 'goto' is now a reserved
word.  This patch adds guidance to extensions.lua.sample and changed
'app.goto("default", "s", 1)' to 'app.['goto']("default", "s", 1)'.

ASTERISK-23844 #close
Reported by: rnewton
Tested by: gtjoseph
Review: https://reviewboard.asterisk.org/r/3627/

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@416581 65c4cc65-6c06-0410-ace0-fbb531ad65f3