docs($http): fixes grunt warning

When you run `grunt package` you get the following warning:

```
warn:    Dangling Links Found in "partials/api/ng/service/$http.html":
 - #overriding-the-default-transformations-per-request,
 - #overriding-the-default-transformations-per-request
```

This change fixes the links, so the warning will no longer appear.
I think we should be able to just use `ng.$http#...` for this instead --- but it basically looks good, will merge it tonight if nobody else beats me to it