update twtxt.net/yarn.social url (#2828)

yarn.social is the main page, twtxt.net is just one of the instances, now

README.md

@@ -545,7 +545,7 @@ Mailing lists servers and mass mailing software - one message to many recipients
 - [Thredded](https://github.com/thredded/thredded) - Forums, feature-rich and simple. `MIT` `Ruby`
 - [Tokumei](https://tokumei.co/) - Anonymous microblogging platform. ([Source Code](https://gitlab.com/tokumei/tokumei)) `ISC` `rc`
 - [twister](http://twister.net.co/) - Fully decentralized P2P microblogging platform leveraging the free software implementations of Bitcoin and BitTorrent protocols. ([Source Code](https://github.com/miguelfreitas/twister-core)) `MIT` `C++`
-- [twtxt.net](https://twtxt.net) - a Self-Hosted, Twitter™-like Decentralised micro-logging platform. No ads, no tracking, your content, your data. ([Source Code](https://git.mills.io/yarnsocial/yarn)) `MIT` `Go`
+- [yarn.social](https://yarn.social) - a Self-Hosted, Twitter™-like Decentralised micro-logging platform. No ads, no tracking, your content, your data. ([Source Code](https://git.mills.io/yarnsocial/yarn)) `MIT` `Go`
 - [Vanilla Forums](https://vanillaforums.org/) - Simple and flexible forum software. ([Source Code](https://github.com/vanilla/vanilla)) `GPL-2.0` `PHP`
 - [Zusam](https://zusam.org) - Free and open-source way to self-host private forums for groups of friends or family. ([Demo](https://demo.zusam.org), [Source Code](https://github.com/zusam/zusam)) `AGPL-3.0` `PHP`