docs(guide): change example controller to properly call greet method on greeter

Author: mghardy

docs/content/guide/di.ngdoc

@@ -92,7 +92,7 @@ dependency lookup responsibility to the injector by declaring the dependencies a
   // And this controller definition
   function MyController($scope, greeter) {
     $scope.sayHello = function() {
-      greeter('Hello World');
+      greeter.greet('Hello World');
     };
   }
 
@@ -231,4 +231,4 @@ of declaring factories is:
     run(['depService', function(depService) {
       ...
     }]);
-</pre>
+</pre>