apB: fixing typo, per getify#138

Author: getify

manuscript/apB.md

@@ -203,7 +203,7 @@ If the *how* of this discussion on monad methods is unclear so far, go back and
 
 It's very common in FP material to cover well-known monads like Maybe. Actually, the Maybe monad is a particular pairing of two other simpler monads: Just and Nothing.
 
-We've already seen Just; Nothing is a monad that holds an empty value. Maybe is a monad that either holds a Just or an Empty.
+We've already seen Just; Nothing is a monad that holds an empty value. Maybe is a monad that either holds a Just or a Nothing.
 
 Here's a minimal implementation of Maybe: