DOC: capitalize Gregorian and fix word order
Gregorian is a proper noun
(https://en.wikipedia.org/wiki/Proleptic_Gregorian_calendar). Also,
"proleptic" is modifying the compound noun "Gregorian epoch" rather than
"Gregorian" modifying a "proleptic epoch".

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

Thanks @erictleung 