Add missing colon (#3274)

Author: AlexanderLukin

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

@@ -1416,7 +1416,7 @@ a#router-link-stub
 :marked
   ### Stubbing the _RouterLink_
 
-  The `RouterLinkStubDirective` contributes substantively to the test
+  The `RouterLinkStubDirective` contributes substantively to the test:
 +makeExample('testing/ts/src/testing/router-stubs.ts', 'router-link', 'testing/router-stubs.ts (RouterLinkStubDirective)')(format='.')
 :marked
   The `host` metadata property wires the click event of the host element (the `<a>`) to the directive's `onClick` method.