Remove confusing instance variable example in cheat sheet

[atallahade]

The mypy cheat sheet documentation provides an example on how to type instance and class attributes. However, it shows that an attribute with a default value in the class body is an instance variable, and I don't think this is correct. I understand that strings are immutable, which makes the example valid in the end, but I don't believe this is a great practice to define instance variables with a value outside of the init method.

Please let me know if you disagree or would prefer to change the comment rather than deleting the example.

[ikonst]

LGTM. I don't see any value in that example, and agree it's a dubious practice especially when people then carry it into mutable types.