Fix comment referring to formerly-above code

Author: dtolnay

library/alloc/src/panic.rs

@@ -2,7 +2,7 @@ use crate::rc::Rc;
 use crate::sync::Arc;
 use core::panic::{RefUnwindSafe, UnwindSafe};
 
-// not covered via the Shared impl above b/c the inner contents use
+// not covered via the Shared impl in libcore b/c the inner contents use
 // Cell/AtomicUsize, but the usage here is unwind safe so we can lift the
 // impl up one level to Arc/Rc itself
 #[stable(feature = "catch_unwind", since = "1.9.0")]