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
 
@@ -196,7 +196,7 @@ later copied into stage2 as well (both the compiler's `libdir` and the
 This `std` is pretty much necessary for any useful work with the compiler.
 Specifically, it's used as the `std` for programs compiled by the newly compiled
 compiler (so when you compile `fn main() { }` it is linked to the last `std`
-compiled with `x.py build --stage 1 library/std`).
+compiled with `x.py build library/std`).
 
 The `rustc` generated by the stage0 compiler is linked to the freshly-built
 `std`, which means that for the most part only `std` needs to be cfg-gated,