Fix README links targets

[arkid15r]

Some of the README.rst's Contributions and License section links looks right on Github but result in 404 on PYPI and RTD platforms. We need to fix it so links would work right everywhere.

[nwithan8]

Working fine for me from PyPi: https://pypi.org/project/holidays/

They're absolute URLs, so shouldn't be a problem.

[arkid15r]

That's right, the issue has been resolved in #1745

Thanks for verifying this!