docs(cheatsheet): update to new syntax

See https://github.com/angular/angular.io/pull/459

Closes #5733

modules/angular2/docs/cheatsheet/forms.md

@@ -5,5 +5,7 @@ Forms
 `import {FORM_DIRECTIVES} from 'angular2/angular2';`
 
 @cheatsheetItem
+syntax:
 `<input [(ng-model)]="userName">`|`[(ng-model)]`
+description:
 Provides two-way data-binding, parsing and validation for form controls.