message: fix description of HASHTABLE_LOOKUP
  HASHTABLE_LOOKUP is a builtin op rather than a custom one.
diff: @@ -137,8 +137,8 @@ Following are the ops supported for using On-Device Smart Reply model:
  * **HASHTABLE_LOOKUP**
  - This is a custom op that uses label id from predict op and looks up the
  - response text from the given label id.
  + This is an op inside TensorFlow Lite that uses label id from predict op and
  + looks up the response text from the given label id.
  ## Further Information
  