doc: fix cppcheck warnings

doc/tutorial/hello_world.c

@@ -1,7 +1,7 @@
 /* StarPU --- Runtime system for heterogeneous multicore architectures.
  *
  * Copyright (C) 2010-2011  Université de Bordeaux
- * Copyright (C) 2010-2011, 2013  CNRS
+ * Copyright (C) 2010-2011, 2013, 2016  CNRS
  *
  * StarPU is free software; you can redistribute it and/or modify
  * it under the terms of the GNU Lesser General Public License as published by
@@ -38,7 +38,7 @@ struct starpu_codelet cl =
 
 void callback_func(void *callback_arg)
 {
-	printf("Callback function (arg %x)\n", callback_arg);
+	printf("Callback function (arg %p)\n", callback_arg);
 }
 
 int main(int argc, char **argv)