Updated `check-dict`.

* Check dict now verifies 'desc:' fields as expected. Square brackets
  are expected to contain source references, except when found inside
  other parentheses or brackes in which case they are expected to NOT
  contain source references. (This is consistent with how they're
  already used in the dictionary, and this code change eliminates a
  warning that has been popping up every time one checks the
  dictionary.)
Added missing word to TKD transcript.

Added missing word "though" in the second sentence of section 6.2.5
"Sentences as Objects". (The word had simply been left out in the
transcript, it is present in the original book.)