Use fewer stages in bootstrapping docs

After rust-lang/rust#73964, they're redundant.

Author: jyn514

src/building/bootstrapping.md

@@ -25,7 +25,7 @@ The `stage2` compiler is the one distributed with `rustup` and all other
 install methods. However, it takes a very long time to build because one must
 first build the new compiler with an older compiler and then use that to
 build the new compiler with itself. For development, you usually only want
-the `stage1` compiler: `x.py build --stage 1 library/std`.
+the `stage1` compiler: `x.py build library/std`.
 
 ## Complications of bootstrapping