docs(testing): fix link (#2514)

Fixed link

Author: Belax8

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

@@ -855,7 +855,7 @@ a(href="#top").to-top Back to top
 
   Notice the `async` call in the `beforeEach`, made necessary by the asynchronous `TestBed.compileComponents` method.
   The `async` function arranges for the tester's code to run in a special _async test zone_
-  that hides the mechanics of asynchronous execution, just as it does when passed to an [_it_ test)(#async).
+  that hides the mechanics of asynchronous execution, just as it does when passed to an [_it_ test](#async).
 
 #compile-components
 :marked