Fix bitsets examples
Fix bitsets examples

You cannot do (A + e) or (A - e) where A is a set and e is an
element. But you can do (A + {e}) or (A - {e}).

Ref:

- https://nim-lang.org/docs/manual.html#types-set-type

This commit also fixes a typo, the missing quote in the intersection
example.

Fixes https://github.com/flaviut/nim-by-example/issues/72.

flaviut/nim-by-example,flaviut/nim-by-example,flaviut/nim-by-example,flaviut/nim-by-example