docs: Typo fix (an > a) (angular#1747)

Author: PatrickMcD

public/docs/ts/latest/guide/template-syntax.jade

@@ -1234,7 +1234,7 @@ figure.image-display
 :marked
   ## Template reference variables
 
-  A **template reference variable** is a reference to an DOM element or directive within a template.
+  A **template reference variable** is a reference to a DOM element or directive within a template.
 
   It can be used with native DOM elements but also with Angular 2 components — in fact, it will work with any custom web component.