mirror docs/recent/pdf/ links to docs/7.8/pdf/

[spdegabrielle]

https://mirror.racket-lang.org/docs/recent/pdf/ links to https://mirror.racket-lang.org/docs/7.8/pdf/

Should be https://mirror.racket-lang.org/docs/8.13/pdf/

I'd change it myself but I can't find the source for mirror.racket-lang.org

[spdegabrielle]

also noticed ; https://mirror.racket-lang.org/docs/pdf/guide.pdf links to version 6.6.