Update subscriptions.md

I was little confused while tried to add this to `Endpoint` because felt to be in this context:
```elixir
use Absinthe.Phoenix.Socket,
    schema: MyApp.Web.Schema
```
I think setup guide can be more clear with adding `UserSocket` filename to the step instruction.

guides/subscriptions.md

@@ -63,7 +63,7 @@ In your `MyAppWeb.Endpoint` module add:
 use Absinthe.Phoenix.Endpoint
 ```
 
-For your socket, different configurations are used depending on what version of
+For your socket, different configurations are used in `MyAppWeb.UserSocket` depending on what version of
 Phoenix you're using.
 
 #### Phoenix 1.3 and 1.4