Update step_05.ngdoc

```_$httpBackend_``` is a cryptic way to name variables. Because of this, even you had to write a comment about this why did you use it that way! This is crazy.

So, just making ```_$httpBackend_``` as ```$httpBackend```, and the local var ```$httpBackend``` to ```$http``` we can easily smooth this complexity.

Elegancy is the key.