Fix minor typo

Author: uchijo

markdown/source_md/a-fistful-of-monads.md

@@ -222,7 +222,7 @@ class Monad m where
 Let's start with the first line.
 It says `class Monad m where`.
 But wait, didn't we say that monads are just beefed up applicative functors?
-Shouldn't there be a class constraint in there along the lines of `class (Applicative m) = > Monad m where` so that a type has to be an applicative functor first before it can be made a monad?
+Shouldn't there be a class constraint in there along the lines of `class (Applicative m) => Monad m where` so that a type has to be an applicative functor first before it can be made a monad?
 Well, there should, but when Haskell was made, it hadn't occurred to people that applicative functors are a good fit for Haskell so they weren't in there.
 But rest assured, every monad is an applicative functor, even if the `Monad` class declaration doesn't say so.