tweak wording

Co-authored-by: varkor <github@varkor.com>

src/librustc_ast_passes/ast_validation.rs

@@ -1132,7 +1132,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                         if matches!(param.kind, GenericParamKind::Const { .. }) {
                             err.note(
                                 "using type defaults and const parameters \
-                                 in the same parameter listing is currently not possible",
+                                 in the same parameter list is currently not permitted",
                             );
                         }
                         err.emit();

src/test/ui/const-generics/defaults/wrong-order.stderr

@@ -4,7 +4,7 @@ error: type parameters with a default must be trailing
 LL | struct A<T = u32, const N: usize> {
    |          ^
    |
-   = note: using type defaults and const parameters in the same parameter listing is currently not possible
+   = note: using type defaults and const parameters in the same parameter list is currently not permitted
 
 warning: the feature `const_generics` is incomplete and may not be safe to use and/or cause compiler crashes
   --> $DIR/wrong-order.rs:1:12