Fix typo in docs.

Fixes #2734.

core/lib/src/rocket.rs

@@ -366,7 +366,7 @@ impl Rocket<Build> {
     ///     "Whoops! Looks like we messed up."
     /// }
     ///
-    /// #[catch(400)]
+    /// #[catch(404)]
     /// fn not_found(req: &Request) -> String {
     ///     format!("I couldn't find '{}'. Try something else?", req.uri())
     /// }