Update double_factorial.cpp

math/double_factorial.cpp

@@ -10,7 +10,7 @@ of a number and is denoted by !! */
 
 uint64_t double_factorial_iterative(uint64_t n) {
   uint64_t res = 1;
-  for ( unsigned long long i = n; i >= 0; i -= 2 ) {
+  for ( uint64_t i = n; i >= 0; i -= 2 ) {
     if (i == 0 || i == 1) return res;
     res *= i;
   }