docs(style-guide): fix typo (#3120)

Author: Lukas Ruebbelke

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

@@ -567,7 +567,7 @@ a#compile-components
   nor any of the `override...` methods. The `TestBed` throws an error if you try.
 
   ### The second synchronous _beforeEach_
-  A _synchronous_ `beforeEach` containing the remaining setup steps follows the asynchonous `beforeEach`.
+  A _synchronous_ `beforeEach` containing the remaining setup steps follows the asynchronous `beforeEach`.
 
 +makeExample('testing/ts/app/banner.component.spec.ts', 'sync-before-each', 'app/banner.component.spec.ts (second beforeEach)')(format='.')
 :marked