Adjust rustc_bootstap.rs ui test

- Fixed test name, it should've been `rustc_bootstrap.rs`, oops.
- Slightly reworded test comment to make it more clear.

Author: jieyouxu

tests/ui/bootstrap/rustc_bootstap.force_stable.stderr renamed to tests/ui/bootstrap/rustc_bootstrap.force_stable.stderr

@@ -1,5 +1,5 @@
-//! Check `RUSTC_BOOTSTRAP`'s behavior in relation to feature stability and what rustc considers
-//! itself to be (stable vs non-stable ).
+//! Check the compiler's behavior when the perma-unstable env var `RUSTC_BOOTSTRAP` is set in the
+//! environment in relation to feature stability and which channel rustc considers itself to be.
 //!
 //! `RUSTC_BOOTSTRAP` accepts:
 //!