fix contract function return type

readme.md

@@ -343,7 +343,7 @@ const predicate = (a) => a > 2
 A contract specifies the obligations and guarantees of the behavior from a function or expression at runtime. This acts as a set of rules that are expected from the input and output of a function or expression, and errors are generally reported whenever a contract is violated.
 
 ```js
-// Define our contract : int -> int
+// Define our contract : int -> boolean
 const contract = (input) => {
   if (typeof input === 'number') return true
   throw new Error('Contract violated: expected int -> int')