fix const_trait unstable message

Author: fee1-dead

compiler/rustc_feature/src/builtin_attrs.rs

@@ -468,7 +468,7 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     // RFC 2632
     gated!(
         const_trait, Normal, template!(Word), WarnFollowing, const_trait_impl,
-        "`const` is a temporary placeholder for marking a trait that is suitable for `const` \
+        "`const_trait` is a temporary placeholder for marking a trait that is suitable for `const` \
         `impls` and all default bodies as `const`, which may be removed or renamed in the \
         future."
     ),