Fix lint example

Author: estebank

compiler/rustc_lint/src/default_could_be_derived.rs

@@ -20,8 +20,8 @@ declare_lint! {
     /// }
     ///
     /// #[deny(default_could_be_derived)]
-    /// impl Default for Foo {
-    ///     fn default() -> Foo {
+    /// impl Default for A {
+    ///     fn default() -> A {
     ///         A {
     ///             b: None,
     ///         }