Switch from bad url to working url (#825)

The previously linked website had redirected to https://golang50shad.es/ which displayed some kind of domain expired page with links to buy domains.
When googling 50 shades of go this url shows up and it seems to work and looks similar/identical to the previous url.

Author: frojd-trond-froding

README.md

@@ -1281,7 +1281,7 @@ Yes! Either way, you should test your Docker images.
 
 ### How can I contribute if I don't know Go?
 
-You don't need to read the language spec and lots of books :-) Go through the [Tour of Go](https://tour.golang.org/welcome/1) and optionally read [50 Shades of Go](http://devs.cloudimmunity.com/gotchas-and-common-mistakes-in-go-golang/) and you'll be ready to contribute!
+You don't need to read the language spec and lots of books :-) Go through the [Tour of Go](https://tour.golang.org/welcome/1) and optionally read [50 Shades of Go](https://golang50shades.com/) and you'll be ready to contribute!
 
 ### What's the best application for Slim?