Fix Progress spelling error in main/pbx.c.

(closes issue ASTERISK-18857)
Reported by: David M
Patches:
      mainpbx-trivial.patch (License #6326) patch uploaded by David M
........

Merged revisions 345219 from http://svn.asterisk.org/svn/asterisk/branches/1.8
........

Merged revisions 345220 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@345221 65c4cc65-6c06-0410-ace0-fbb531ad65f3