Rename KEY_KP_SUBSTRACT to KEY_KP_SUBTRACT
Created by: akien-mga
The former name was incorrect in English, though for us latin lovers it's an understandable mistake. Second part of and closes #3626 (closed).
I chose not to bother keeping the old name and adding deprecating warnings; I'll admit it's partly because I have no clue how to do that without adding a lot of clutter :p But also the issue is pretty easy to fix for the couple users that might be affected.