refactoring: changed (prime - 1) to (prime - 2) in multi_by_prime fonction

src/multiplication.c

@@ -96,7 +96,7 @@ int multi_using_modulo(int num1, int num2, int modulo) {
 }
 
 int multi_by_prime(int num, int prime) {
-    return num * (prime - 1) + num;
+    return num * (prime - 2) + num;
 }
 
 int multi_using_increment(int num1, int num2) {