docs(guide/controller): clarify that controllers are defined **by** a constructor function

A controller is a instantiated object created **from** a constructor function.
It was not accurate to describe a Controller **as** a constructor function.

Closes angular#11888

Author: niteshthakur

docs/content/guide/controller.ngdoc

@@ -5,7 +5,7 @@
 
 # Understanding Controllers
 
-In Angular, a Controller is a JavaScript **constructor function** that is used to augment the
+In Angular, a Controller is defined by a JavaScript **constructor function** that is used to augment the
 {@link scope Angular Scope}.
 
 When a Controller is attached to the DOM via the {@link ng.directive:ngController ng-controller}