Added dyn to Type::TraitObject docs

The old syntax is now a compile error, and it makes it difficult for users to search when they are looking for the keyword "dyn" on the documentation.

src/ty.rs

@@ -54,7 +54,7 @@ ast_enum_of_structs! {
  /// A dynamically sized slice type: `[T]`.
  Slice(TypeSlice),
 
- /// A trait object type `Bound1 + Bound2 + Bound3` where `Bound` is a
+ /// A trait object type `dyn Bound1 + Bound2 + Bound3` where `Bound` is a
  /// trait or a lifetime.
  TraitObject(TypeTraitObject),
 
@@ -244,7 +244,7 @@ ast_struct! {
 }
 
 ast_struct! {
- /// A trait object type `Bound1 + Bound2 + Bound3` where `Bound` is a
+ /// A trait object type `dyn Bound1 + Bound2 + Bound3` where `Bound` is a
  /// trait or a lifetime.
  ///
  /// *This type is available only if Syn is built with the `"derive"` or