docs(guide/forms): update example

Right now, non-integers such as 'aawefwae' are valid.
This ensures that only integers are valid. Hopefully that makes the example more powerful.

Closes #5501

docs/content/guide/forms.ngdoc

@@ -235,7 +235,7 @@ In the following example we create two directives.
 <script>
   var app = angular.module('form-example1', []);
 
-  var INTEGER_REGEXP = /^\-?\d*$/;
+  var INTEGER_REGEXP = /^\-?\d+$/;
   app.directive('integer', function() {
     return {
       require: 'ngModel',