Update slider-class.scrbl

Remove double negation, as suggested by sorawee.

gui-doc/scribblings/gui/slider-class.scrbl

@@ -44,9 +44,9 @@ If @racket[label] is a string, it is used as the label for the slider.
 
 The @racket[min-value] and @racket[max-value] arguments specify the
  range of the slider, inclusive. The @racket[init-value] argument
- optionally specifies the slider's initial value. If the sequence
+ optionally specifies the slider's initial value. The sequence
  [@racket[min-value], @racket[init-value], @racket[max-value]]
- is not non-decreasing, @|MismatchExn|.
+ must be non-decreasing. Otherwise, @|MismatchExn|.
 
 The @racket[callback] procedure is called (with the event type
  @indexed-racket['slider]) when the user changes the slider's value.