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
```