Minor README.md typo fix "usefull" > "useful".

Author: StevenMaude

README.md

@@ -2,7 +2,7 @@ SL(1): Cure your bad habit of mistyping
 =======================================
 
 SL (Steam Locomotive) runs across your terminal when you type "sl" as
-you meant to type "ls". It's just a joke command, and not usefull at
+you meant to type "ls". It's just a joke command, and not useful at
 all.
 
 Copyright 1993,1998,2014 Toyoda Masashi ([email protected])