Update fork_101.md (#150)

exercises/os/fork_101.md

@@ -4,12 +4,12 @@ Answer the questions given the following program (without running it):
 
 ```
 #include<stdio.h>
-
+#include <unistd.h> 
 int main()
 {
-fork();
-printf("\nyay\n");
-return 0;
+  fork();
+  printf("\nyay\n");
+  return 0;
 }
 ```