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/
 
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Merged revisions 416581 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 416582 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@416589 65c4cc65-6c06-0410-ace0-fbb531ad65f3