docs(ionSlideBox): fix active-slide docs

Closes ionic-team#4210

js/angular/directive/slideBox.js

@@ -33,7 +33,7 @@
  * @param {boolean=} show-pager Whether a pager should be shown for this slide box. Accepts expressions via `show-pager="{{shouldShow()}}"`. Defaults to true.
  * @param {expression=} pager-click Expression to call when a pager is clicked (if show-pager is true). Is passed the 'index' variable.
  * @param {expression=} on-slide-changed Expression called whenever the slide is changed.  Is passed an '$index' variable.
- * @param {expression=} active-slide Model to bind the current slide to.
+ * @param {expression=} active-slide Model to bind the current slide index to.
  */
 IonicModule
 .directive('ionSlideBox', [