Update RDG2

Author: Kobzol

src/doc/rustc-dev-guide/README.md

@@ -1,6 +1,7 @@
 [![CI](https://github.com/rust-lang/rustc-dev-guide/actions/workflows/ci.yml/badge.svg)](https://github.com/rust-lang/rustc-dev-guide/actions/workflows/ci.yml)
 
 # Update RDG
+# Update RDG2
 
 This is a collaborative effort to build a guide that explains how rustc
 works. The aim of the guide is to help new contributors get oriented