Improved Anet A8 Filament Guide

I have made a few improvements to the original [Anet A8 Filament Guide](https://www.thingiverse.com/thing:1795148). This part was made for the new Anet A8 and I have no clue if it will work on the old one. Here are the changes:

* Scaling has been fixed, so you no longer need to enter weird values in your slicer.
* The guide head was moved slightly downwards to allow for more clearance for a screwdriver.
* The guide head has been properly centered above the extruder opening.

I have also included the Blender files in case you want to edit it further.