Change stage0 cfg_attr to bootstrap

Author: flip1995

src/stabilization_guide.md

@@ -125,7 +125,7 @@ writing, the next stable release (i.e. what is currently beta) was
 Next search for the feature string (in this case, `pub_restricted`)
 in the codebase to find where it appears. Change uses of
 `#![feature(XXX)]` from the `libstd` and any rustc crates to be
-`#![cfg_attr(stage0, feature(XXX))]`. This includes the feature-gate
+`#![cfg_attr(bootstrap, feature(XXX))]`. This includes the feature-gate
 only for stage0, which is built using the current beta (this is
 needed because the feature is still unstable in the current beta).