Updated README.md
Removed extra white spaces and clarified that NAN stands for Not a Number.

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

@jaydeepborkar if you want to contribute to pandas, please consider the issues marked as "good first issue", not arbitrary changes.