fix(guide/form): fix a broken link (#2498)

Author: alirezamirian

public/docs/ts/latest/guide/dependency-injection.jade

@@ -362,7 +362,7 @@ block ctor-syntax
 
 .l-sub-section
   :marked
-    Learn more in [Testing](../testing/index.html).
+    Learn more in [Testing](./testing.html).
 
 :marked
   ### When the service needs a service