Update recommended diagram slice size

Author: ben

CONTRIBUTING.md

@@ -25,7 +25,7 @@ To add a figure:
 
 1. Add a page to the sketchbook. Try to use the included symbols wherever possible.
 1. Add a "slice" to your page. Give it a name that matches the destination PNG filename, relative from the root of the source directory.
-1. Make sure your slice is set to export at 3x size.
+1. Make sure your slice is set to export at "800w".
 
 
 ## Translations