DOC: modify typos in Contributing section
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

I participated in an open source sprint day today. This is my first attempt to request a PR to contribute to open source. I made changes to fix a few minor typos.
@alyssafuward Thanks a lot! Thank you for the support! This totally made my night! @alyssafuward : Our pleasure!  We hope that you'll continue to contribute more to our repository (and to open source in general) in the future!