Say "bootstrap" instead of "rustbuild"; the latter is not explained anywhere and is not much more clear.

Author: jyn514

src/building/bootstrapping.md

@@ -54,7 +54,7 @@ and its associated dynamic libraries,
 which `x.py` will download for you.
 (You can also configure `x.py` to use something else.)
 
-The stage0 compiler is then used only to compile `rustbuild`, `std`, and `rustc`.
+The stage0 compiler is then used only to compile `src/bootstrap`, `std`, and `rustc`.
 When compiling `rustc`, the stage0 compiler uses the freshly compiled `std`.
 There are two concepts at play here:
 a compiler (with its set of dependencies)