Update index.md

Maybe I'm a fool, but the tag/branch release0.1 doesn't seem to exist when I try to clone.

    danking@beverly # git clone git@github.com:JuliaLang/julia.git -b 'release0.1' julia-0.1
    Cloning into 'julia-0.1'...
    Enter passphrase for key '/home/danking/.ssh/id_rsa': 
    fatal: Remote branch release0.1 not found in upstream origin

I, however, can check out the tag for v0.1.2. Is this the intended tag? This pull request updates the downloads page to use the v0.1.2 tag instead of release0.1.