Fix Solarus Launcher Link in README
One of the links in the README didn't work. Links to files in a repository are kind of weird unless you use a relative link from another file in the repository.
I tested it as best I could by going to my local version on GitLab and trying it there. That version is at: https://gitlab.com/Cluedrew/solarus/-/tree/fix-readme (I'll leave the branch there until this is merged).