[DOC beta] Fix ClassMixin.reopen docs.

Before:

![image](https://www.monosnap.com/image/6HQ4hKotoI6f3VhXaJczYnApH.png)

After:

![image](https://www.monosnap.com/image/625WeLaZv9fXCShEwl3XLICqE.png)