Outdated url to Hunspell

I happened to notice that the link to Hunspell in our documentation goes to the
hunspell sourceforge page last updated in 2015. The project has since moved to
Github [0]https://hunspell.github.io/ with hunspell.sourceforge.net redirecting there, I'm not sure
exactly when but Wikipedia updated their link entry in 2016 [1]https://en.wikipedia.org/w/index.php?title=Hunspell&diff=704306462&oldid=697230645 so it seems
about time we do too.

Unless objected to I'll apply the attached to master with the doc/ hunk to all
supported branches.

--
Daniel Gustafsson https://vmware.com/

[0]: https://hunspell.github.io/
[1]: https://en.wikipedia.org/w/index.php?title=Hunspell&diff=704306462&oldid=697230645