message: update _default
diff: @@ -18,7 +18,7 @@ class arg_num_threads : public int_argument {
  #else
  _validity = "num_threads == 1";
  #endif
  - _default = boost::lexical_cast<std::string>(stan::math::internal::get_num_threads());
  + _default = "1 or the value of the STAN_NUM_THREADS environment variable if set.";
  _default_value = stan::math::internal::get_num_threads();
  _good_value = 1.0;
  _bad_value = -2.0;
  