codec_adpcm: Change description of codec "ADPCM" to "Dialogic ADPCM"

Technically, ADPCM is a method that can be applied to several codecs.
Asterisk's ADPCM codec is the Dialogic ADPCM or VOX codec.

See http://en.wikipedia.org/wiki/Dialogic_ADPCM for more information
about said codec.

Review: https://reviewboard.asterisk.org/r/3744

patches:
  rb3744.patch uploaded by dennis.guse (License 6513)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@418738 65c4cc65-6c06-0410-ace0-fbb531ad65f3