GraphCopy: fix Windows export_clean_symbol for __Nil and __None (again)

Ghost User
2021-05-21T10:15:07+02:00

In !19 I did not take the two extra `__` characters for `__Nil` and `__None` into account. The sizes of the strings should be 5 and 6, not 3 and 4.

This probably causes https://gitlab.com/clean-and-itasks/abc-interpreter/-/issues/113, but I have no way to test at the moment.