docs(guide/concepts): insert comma

Closes #15166

Author: JonyD

docs/content/guide/concepts.ngdoc

@@ -155,7 +155,7 @@ More accurately, the file specifies a constructor function that will be used to
 controller instance. The purpose of controllers is to expose variables and functionality to
 expressions and directives.
 
-Besides the new file that contains the controller code we also added an
+Besides the new file that contains the controller code, we also added an
 {@link ng.directive:ngController `ng-controller`} directive to the HTML.
 This directive tells Angular that the new `InvoiceController` is responsible for the element with the directive
 and all of the element's children.