fix: print error on stderr

src/print.c

@@ -2,14 +2,15 @@
 
 #include <stdarg.h>
 #include <stdio.h>
+#include <unistd.h>
 
 void print_err(const char *format, ...)
 {
     va_list args;
 
-    puts("ft_ping: ");
+    dprintf(2, "ft_ping: ");
     va_start(args, format);
-    vprintf(format, args);
+    vdprintf(2, format, args);
     va_end(args);
-    putchar('\n');
+    write(2, "\n", 1);
 }