bst_valid_challenge's one test case is wrong

hi, I noticed that test case below is not a valid binary search tree, it might be valid if we delete the node 6, but in your challenge notebook marked it as valid. I'm not sure I should delete the node 6 and send a pr or just submit an issue, so I just submit an issue. Valid:

      5
    /   \
   5     8
  / \   /
 4   6 7