docs(guide/Dependency Injection): fix angular.injector arguments list

The original file included a code sample using `angular.injector(['myModule', 'ng'])`,
which appears to be incorrect when trying to retrieve anything attached to `myModule`.
Reversing the args fixes this.

Closes angular#12292

Author: adamsea

docs/content/guide/di.ngdoc

@@ -293,7 +293,7 @@ Create a new injector that can provide components defined in our `myModule` modu
 `greeter` service from the injector. (This is usually done automatically by angular bootstrap).
 
 ```js
-var injector = angular.injector(['myModule', 'ng']);
+var injector = angular.injector(['ng', 'myModule']);
 var greeter = injector.get('greeter');
 ```