Remove extraneous brackets
Trivial documentation changes

Dictionary should have one set of brackets (as seen below in documentation example `{'a': np.float64, 'b': np.int32}`).

Brackets don't make sense around `'w' or 'a'`.

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
