error: fix example of using TracedError with error kind pattern (#635)

tracing-error/src/error.rs

@@ -17,11 +17,26 @@ struct Erased;
 ///
 /// ```rust,compile_fail
 /// #[derive(Debug, thiserror::Error)]
+/// enum Kind {
+///     // ...
+/// }
+///
+/// #[derive(Debug)]
 /// pub struct Error {
 ///     source: TracedError<Kind>,
 ///     backtrace: Backtrace,
 /// }
 ///
+/// impl std::error::Error for Error {
+///     fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+///         self.source.source()
+///     }
+///
+///     fn backtrace(&self) -> Option<&Backtrace> {
+///         Some(&self.backtrace)
+///     }
+/// }
+///
 /// impl fmt::Display for Error {
 ///     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
 ///         fmt::Display::fmt(&self.source, fmt)