Update fork_102.md (#151)

This include is needed

```
#include <unistd.h> 
```

exercises/os/fork_102.md

@@ -4,13 +4,14 @@ Answer the questions given the following program (without running it):
 
 ```
 #include<stdio.h>
+#include <unistd.h> 
 
 int main()
 {
-fork();
-fork();
-printf("\nyay\n");
-return 0;
+  fork();
+  fork();
+  printf("\nyay\n");
+  return 0;
 }
 ```