Fix typos in comments for NavControllerBase and ViewController

src/navigation/nav-controller-base.ts

@@ -686,7 +686,7 @@ export class NavControllerBase extends Ion implements NavController {
   _viewsWillLifecycles(enteringView: ViewController, leavingView: ViewController) {
     if (enteringView || leavingView) {
       this._zone.run(() => {
-        // Here, the order is important. WillLeave must called before WillEnter.
+        // Here, the order is important. WillLeave must be called before WillEnter.
         leavingView && this._willLeave(leavingView, !enteringView);
         enteringView && this._willEnter(enteringView);
       });

src/navigation/view-controller.ts

@@ -463,7 +463,7 @@ export class ViewController {
 
   /**
    * @private
-   * The view has is about to leave and no longer be the active view.
+   * The view is about to leave and no longer be the active view.
    */
   _willLeave(willUnload: boolean) {
     this.willLeave.emit(null);