Replace tilde '~' with escaped value %7E in URL description

e.g.

[https://people.freebsd.org/~wosch/build-time/buildworld/]

>

[https://people.freebsd.org/%7Ewosch/build-time/buildworld/]

This is a well known old bug in the hugo framework, see
https://github.com/wowchemy/wowchemy-hugo-themes/issues/87