Add missing link to Stack for Podcast 68 (#539)

Thanks!

Author: mpilgrem

podcast/68/links.markdown

@@ -1,10 +1,11 @@
 + [Perl6](https://en.wikipedia.org/wiki/Raku_(programming_language))
 + [Pugs](https://en.wikipedia.org/wiki/Pugs_(compiler))
-+ [Yesod](https://www.yesodweb.com/)	
-+ [Django (python)](https://www.djangoproject.com/) 
-+ [FP Complete](https://tech.fpcomplete.com/) 
++ [Yesod](https://www.yesodweb.com/)
++ [Django (python)](https://www.djangoproject.com/)
++ [FP Complete](https://tech.fpcomplete.com/)
 + [Intero](https://hackage.haskell.org/package/intero)
-+ [GHCUp](https://www.haskell.org/ghcup/) 
++ [Stack](https://docs.haskellstack.org/en/stable/)
++ [GHCUp](https://www.haskell.org/ghcup/)
 + [Stackage](https://www.stackage.org/)
 + [STM](https://hackage.haskell.org/package/stm)
 + [Rust](https://www.rust-lang.org/)
@@ -14,7 +15,7 @@
 + [Kazu Yamamoto](https://github.com/kazu-yamamoto)
 + [Scotty](https://hackage.haskell.org/package/scotty)
 + [Spock](https://hackage.haskell.org/package/Spock)
-+ [Yesod book](https://www.yesodweb.com/book) 
++ [Yesod book](https://www.yesodweb.com/book)
 + [Lazy I/O](https://book.realworldhaskell.org/read/io.html)
 + [Serde (Rust)](https://serde.rs/)
 + [Clap](https://docs.rs/clap/latest/clap/)