app_confbridge: Fix confbridge.conf dsp_talking_threshold option setting wrong parameter.

Fixed copy pasta error.

ASTERISK-23545 #close
Reported by: John Knott
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Merged revisions 411944 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@411945 f38db490-d61c-443f-a65b-d21fe96a405b