update MSYS2 link in README
update MSYS2 link in README

Now https://msys2.github.io/ redirects to https://www.msys2.org/, so
the README might just link to that immediately.

aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust