Update factorial

Add negative input validation to factorial implementation

Author: AllThingsSmitty

README.md

@@ -144,7 +144,10 @@ console.log(fibonacci(6)); // Output: 8
 
 ```js
 function factorial(n) {
-  if (n === 0) return 1;
+  if (n < 0) throw new RangeError('Factorial is not defined for negative numbers');
+
+  if (n === 0 || n === 1) return 1;
+
   return n * factorial(n - 1);
 }