rename scroll in tooltip

Author: andrewseguin

src/lib/tooltip/tooltip.ts

@@ -95,7 +95,7 @@ export class MdTooltip implements OnInit, OnDestroy {
   }
 
   constructor(private _overlay: Overlay,
-              private _scroll: ScrollDispatcher,
+              private _scrollDispatcher: ScrollDispatcher,
               private _elementRef: ElementRef,
               private _viewContainerRef: ViewContainerRef,
               private _ngZone: NgZone,
@@ -104,7 +104,7 @@ export class MdTooltip implements OnInit, OnDestroy {
   ngOnInit() {
     // When a scroll on the page occurs, update the position in case this tooltip needs
     // to be repositioned.
-    this._scroll.scrolled().subscribe(() => {
+    this._scrollDispatcher.scrolled().subscribe(() => {
       if (this._overlayRef) {
         this._overlayRef.updatePosition();
       }