docs($window): move use of $window to controller

Move use of `$window` from template to controller, because accessing `$window`
in expressions is now disallowed and doesn't work.

Closes #5110

src/ng/window.js

@@ -20,13 +20,15 @@
      <doc:source>
        <script>
          function Ctrl($scope, $window) {
-           $scope.$window = $window;
            $scope.greeting = 'Hello, World!';
+           $scope.doGreeting = function(greeting) {
+               $window.alert(greeting);
+           };
          }
        </script>
        <div ng-controller="Ctrl">
          <input type="text" ng-model="greeting" />
-         <button ng-click="$window.alert(greeting)">ALERT</button>
+         <button ng-click="doGreeting(greeting)">ALERT</button>
        </div>
      </doc:source>
      <doc:scenario>