docs: Update 02_directives.md

Closes #3191

modules/angular2/docs/core/02_directives.md

@@ -67,7 +67,7 @@ Here is a trivial example of a tooltip decorator. The directive will log a toolt
     'text: tooltip'          |  - DOM element tooltip property should be
   ],                         |    mapped to the directive text property.
   host: {                    | List which events need to be mapped.
-    (mouseover): 'show()'    |  - Invoke the show() method every time
+    '(mouseover)': 'show()'    |  - Invoke the show() method every time
   }                          |    the mouseover event is fired.
 })                           |
 class Form {                 | Directive controller class, instantiated