Recommend stage1, not stage2

Author: jyn514

summarize/Readme.md

@@ -80,9 +80,9 @@ 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 --stage 2
+$ ./x.py build
 # This will take a while...
-$ rustup toolchain link mytoolchain build/x86_64-unknown-linux-gnu/stage2
+$ rustup toolchain link mytoolchain build/x86_64-unknown-linux-gnu/stage1
 ```
 
 Where `mytoolchain` is the name of your custom toolchain. Now we do more or less the same