[DOC] Update sample callback of `rb_objspace_each_objects`

* refine liveness check
* fix missing closing brace

gc.c

@@ -3944,14 +3944,16 @@ objspace_each_objects_try(VALUE arg)
  *
  * This is a sample callback code to iterate liveness objects:
  *
- *   int
- *   sample_callback(void *vstart, void *vend, int stride, void *data) {
- *     VALUE v = (VALUE)vstart;
- *     for (; v != (VALUE)vend; v += stride) {
- *       if (RBASIC(v)->flags) { // liveness check
- *       // do something with live object 'v'
- *     }
- *     return 0; // continue to iteration
+ *   static int
+ *   sample_callback(void *vstart, void *vend, int stride, void *data)
+ *   {
+ *       VALUE v = (VALUE)vstart;
+ *       for (; v != (VALUE)vend; v += stride) {
+ *           if (!rb_objspace_internal_object_p(v)) { // liveness check
+ *               // do something with live object 'v'
+ *           }
+ *       }
+ *       return 0; // continue to iteration
  *   }
  *
  * Note: 'vstart' is not a top of heap_page.  This point the first