spelling: useful

Author: jsoref

test/auto/injectorSpec.js

@@ -944,7 +944,7 @@ describe('injector', function() {
     });
 
 
-    it('should throw usefull error on wrong argument type]', function() {
+    it('should throw useful error on wrong argument type]', function() {
       expect(function() {
         $injector.invoke({});
       }).toThrowMinErr('ng', 'areq', 'Argument \'fn\' is not a function, got Object');