docs: fix broken link on testing guide (#2887)

Author: ashishg-qburst

public/docs/ts/latest/guide/testing.jade

@@ -1061,7 +1061,7 @@ a(href="#top").to-top Back to top
   Testing the actual `DashboardComponent` seemed daunting because it injects the `Router`.
 +makeExample('testing/ts/app/dashboard/dashboard.component.ts', 'ctor', 'app/dashboard/dashboard.component.ts (constructor)')(format='.')
 :marked
-  It also injects the `HeroService` but faking that is a [familiar story](#component-with-async-servic).
+  It also injects the `HeroService` but faking that is a [familiar story](#component-with-async-service).
   The `Router` has a complicated API and is entwined with other services and application pre-conditions.
 
   Fortunately, the `DashboardComponent` isn't doing much with the `Router`