fix typo

Co-authored-by: Ralf Jung <post@ralfj.de>

Author: BennoLossin

compiler/rustc_ast/src/ast.rs

@@ -2488,7 +2488,7 @@ pub enum TyKind {
     ImplTrait(NodeId, #[visitable(extra = BoundKind::Impl)] GenericBounds),
     /// No-op; kept solely so that we can pretty-print faithfully.
     Paren(Box<Ty>),
-    /// A `field_of` expression (e.g., `builtin # field_of(Struct, field)`).
+    /// A `field_of` expression (e.g., `builtin # field_of(Struct, fields)`).
     ///
     /// Usually not written directly in user code but
     /// indirectly via the macro `core::field::field_of!(...)`.