qapi: Fix the virtual walk example in visitor.h's big comment

Call visit_check_list().  Missed in commit a4a1c70 "qapi: Make
input visitors detect unvisited list tails".

Drop an irrelevant error_propagate() while there.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200424084338.26803-3-armbru@redhat.com>

include/qapi/visitor.h

@@ -215,14 +215,16 @@
  *      goto outlist;
  *  }
  * outlist:
+ *  if (!err) {
+ *      visit_check_list(v, &err);
+ *  }
  *  visit_end_list(v, NULL);
  *  if (!err) {
  *      visit_check_struct(v, &err);
  *  }
  * outobj:
  *  visit_end_struct(v, NULL);
  * out:
- *  error_propagate(errp, err);
  *  visit_free(v);
  * </example>
  */