Remove "Writing an OS in Rust (first edition)"

README.md

@@ -254,8 +254,7 @@
 * [**C**: _Operating systems development for Dummies_](https://medium.com/@lduck11007/operating-systems-development-for-dummies-3d4d786e8ac)
 * [**C++**: _Write your own Operating System_](https://www.youtube.com/playlist?list=PLHh55M_Kq4OApWScZyPl5HhgsTJS9MZ6M) [video]
 * [**C++**: _Writing a Bootloader_](http://3zanders.co.uk/2017/10/13/writing-a-bootloader/)
-* [**Rust**: _Writing an OS in Rust (first edition)_](https://os.phil-opp.com/first-edition/)
-* [**Rust**: _Writing an OS in Rust (second edition)_](https://os.phil-opp.com/)
+* [**Rust**: _Writing an OS in Rust_](https://os.phil-opp.com/)
 
 #### Build your own `Physics Engine`