DOC: replaced 'Good as first PR' with 'good first issue'
- [NA] closes #xxxx
- [NA] tests added / passed
- [NA] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [NA] whatsnew entry

Was browsing https://github.com/pandas-dev/pandas/tree/master/doc and it seemed the 'Good as first PR' had no open issues, whereas 'good first issue'  has quite a few open issues.
I'm guessing the 'Good as first PR' label was not well used/got renamed to 'good first issue' label?
