docs(02_directives.md): Change the description of bind property

Also add attribute open to the html Closes angular#767

Author: SekibOmazic

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

@@ -114,7 +114,7 @@ Example of a component:
  selector: 'pane', | CSS selector on <pane> element
  bind: { | List which property need to be bound
  'title': 'title', | - title mapped to component title
- 'open': 'open' | - open mapped to component title
+ 'open': 'open' | - open attribute mapped to component's open property
  }, |
 }) |
 @Template({ | Template annotation
@@ -154,7 +154,7 @@ class Pane { | Component controller class
 
 Example of usage:
 ```
-<pane #pane title="Example Title">
+<pane #pane title="Example Title" open="true">
  Some text to wrap.
 </pane>
 <button (click)="pane.toggle()">toggle</button>