message: Fix comment description on crate example
  This tiny PR aligns the comment on the crate example to align to the
  comment that is in README.md.
diff: //! text(self.value).size(50),
  //!
  //! // The decrement button. We tell it to produce a
  +//! // `DecrementPressed` message when pressed
  //! button("-").on_press(Message::DecrementPressed),
  //! ]
  //! }
  