doc: update learn.vyperlang.org hyperlink (#2815)

changed vyper.fun link to https://learn.vyperlang.org/

README.md

@@ -16,7 +16,7 @@ See [Installing Vyper](http://vyper.readthedocs.io/en/latest/installing-vyper.ht
 See [Tools and Resources](https://github.com/vyperlang/vyper/wiki/Vyper-tools-and-resources) for an additional list of framework and tools with vyper support.
 See [Documentation](http://vyper.readthedocs.io/en/latest/index.html) for the documentation and overall design goals of the Vyper language.
 
-See [Vyper.fun](https://vyper.fun?ref=vyperlang) for **learning Vyper by building a Pokémon game**. 
+See [Learn.Vyperlang.org](https://learn.vyperlang.org/) for **learning Vyper by building a Pokémon game**. 
 
 **Note: Vyper is beta software, use with care**