Fix formula (#206)

The current version of the formula incorrectly handles the factorial(0) and causes an infinite loop

03_recursion/ES6/03_factorial.js

@@ -4,7 +4,7 @@
  * @returns {number} Result
  */
 const fact = x => {
-  if (x === 1) return 1;
+  if (x === 0) return 1;
   return x * fact(x - 1);
 };