docs: typo (#87)

docs/effectful_but_side_effect_free.md

@@ -56,7 +56,7 @@ lookup({'key': '42'}, 'key').and_then(maybe_is_42)  # Just('42')
 ```
 (For those with previous functional programming experience, `and_then` is the [bind](https://en.wikipedia.org/wiki/Monad_(functional_programming)#Overview) operation of `Maybe`)
 
-`pfun.maybe.Maybe` is in fact just a [type-alias](https://docs.python.org/3/library/typing.html#type-aliases) for `Union[Just[TypeVar('A'), Nothing]]`. This means that your type checker can figure out when you're dealing with one or the other using either `__bool__` or `isinstance`, just like when using `Optional`:
+`pfun.maybe.Maybe` is in fact just a [type-alias](https://docs.python.org/3/library/typing.html#type-aliases) for `Union[Just[TypeVar('A')], Nothing]`. This means that your type checker can figure out when you're dealing with one or the other using either `__bool__` or `isinstance`, just like when using `Optional`:
 ```python
 value = lookup(some_dict, 'key')
 if value: