docs(reactive-forms): fix broken link API (#3224)

Author: wKoza

public/docs/ts/latest/guide/reactive-forms.jade

@@ -250,7 +250,7 @@ a#essentials
   ### Essential form classes
   It may be helpful to read a brief description of the core form classes.
 
-  * [_AbstractControl_](../api/forms/index/AbstractControl-class.html-class.html "API Reference: AbstractControl")
+  * [_AbstractControl_](../api/forms/index/AbstractControl-class.html "API Reference: AbstractControl")
   is the abstract base class for the three concrete form control classes: 
   `FormControl`, `FormGroup`, and `FormArray`.
   It provides their common behaviors and properties, some of which are _observable_.