Update README for changes in x.py defaults

See rust-lang/rust#73964.

The choices are either to link to `stage1` with rustup or build stage 2; this recommends to build stage2 to be consistent with the existing docs.

Author: jyn514

summarize/Readme.md

@@ -80,7 +80,7 @@ You can also profile your own custom build of rustc. First you'll have to clone
 
 ```bash
 $ git clone https://github.com/rust-lang/rust.git
-$ ./x.py build
+$ ./x.py build --stage 2
 # This will take a while...
 $ rustup toolchain link mytoolchain build/x86_64-unknown-linux-gnu/stage2
 ```