[5.6] Fix return docblock for response helper

[j3j5]

Running larastan (a static code analyzer) I got this error

Call to an undefined method Illuminate\Contracts\Routing\ResponseFactory|::header()

on a similar piece of code to this:

response('content')->header("my header")

After some digging I realized the docblock isn't entirely correct since the helper can only return an instance of Illuminate\Contracts\Routing\ResponseFactory when called empty or an instance of Illuminate\Http\Response when given some content, hence the fix for the docblock.

Since it's a change on a docblock and not actual code it shouldn't break anything.