[DOC] Update `rb_gc_mark_locations` doc

The documentation says that the `end` pointer will be marked
but looking at the source, that is not the case.

include/ruby/internal/intern/gc.h

@@ -71,7 +71,7 @@ RBIMPL_ATTR_NONNULL((1))
  *              addressable.
  * @param[out]  start  Pointer to an array of objects.
  * @param[out]  end    Pointer that terminates the array of objects.
- * @post        Objects from `start` to `end`, both inclusive, are marked.
+ * @post        Objects from `start` (included) to `end` (excluded) are marked.
  *
  * @internal
  *