docs(testing): fix minor typo in DashboardHeroComponent (#2518)

Fixed minor typo in DashboardHeroComponent

Author: Belax8

public/docs/_examples/testing/ts/app/dashboard/dashboard-hero.component.spec.ts

@@ -19,7 +19,7 @@ describe('DashboardHeroComponent when tested directly', () => {
   let heroEl: DebugElement;
 
   // #docregion setup, compile-components
-  // asynch beforeEach
+  // async beforeEach
   beforeEach( async(() => {
     TestBed.configureTestingModule({
       declarations: [ DashboardHeroComponent ],