message: Mention `target_cost` in book (related to
diff: @@ -78,12 +78,19 @@ let res = Executor::new(cost, solver)
  // In this particular case, the state exposed is of type `IterState`.
  // The documentation of `IterState` shows how this struct can be
  // manipulated.
  + // Population based solvers use `PopulationState` instead of
  + // `IterState`.
  .configure(|state|
  state
  - // Set initial parameters
  + // Set initial parameters (depending on the solver,
  + // this may be required)
  .param(init_param)
  // Set maximum iterations to 10
  + // (optional, set to `std::u64::MAX` if not provided)
  .max_iters(10)
  + // Set target cost. The solver stops when this cost
  + // function value is reached (optional)
  + .target_cost(0.0)
  )
  // run the solver on the defined problem
  .run()?;
  