Default Ender 3 fan duct

I take no credit for this part. Model was taken from the solidworks assembly found here:

https://github.com/Creality3DPrinting/Ender-3/issues/1#issuecomment-413782416

All credit goes to Github user Sleeepy and are in the repo from Creality themselves.

I found the model and printed it off after breaking the part when trying out an alternate fan duct.