Add pre-condition for factorial()

exercises/solution-01.md

@@ -6,10 +6,13 @@
 import scala.annotation.tailrec
 
 def factorial(n: Int) =
+  require(n >= 0)
+
   @tailrec
   def loop(n: Int, acc: Int): Int =
     if n == 0 then acc
     else loop(n - 1, acc * n)
+  
   loop(n, 1)
 ```