Add a fn main() {} to a doctest to prevent the test from being wrapped in a fn main() {} body

compiler/rustc_lint/src/opaque_hidden_inferred_bound.rs

@@ -46,6 +46,8 @@ declare_lint! {
     /// fn test() -> impl Trait<Assoc = Tait> {
     ///     42
     /// }
+    ///
+    /// fn main() {}
     /// ```
     ///
     /// {{produces}}