DOC: fixed typo
 - [ ] closes #xxxx
 - [ ] tests added / passed
 - [ ] passes ``git diff upstream/master | flake8 --diff``
 - [ ] whatsnew entry


@smsaladi thanks

@sinhrks BTW, when you such a PR and label it, you are certainly welcome to just merge directly as well (eg for this one with only small chang in the tutorial docs, you don't necessarily have to wait until travis passes) ok, i've waited for tavis:)