Correct Documentation For DEC Function

Correct Documentation For DEC Function

The documentation for DEC in func_math.c was incorrect.  Looks like a copy and
paste error.

(Closes issue ASTERISK-20095)
Reported by: Billy Chia
Tested by: Michael L. Young
Patches:
    func_math.patch uploaded by Billy Chia (license 6381)
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Merged revisions 369970 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 369971 from http://svn.asterisk.org/svn/asterisk/branches/10

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@369974 65c4cc65-6c06-0410-ace0-fbb531ad65f3