message: docs: fix broken link in README
  This link had the wrong reference and didn't render.
diff: @@ -42,7 +42,7 @@ idiomatic `tracing`.)
  In order to record trace events, executables have to use a `Subscriber`
  implementation compatible with `tracing`. A `Subscriber` implements a way of
  collecting trace data, such as by logging it to standard output.
  -[`tracing_subscriber`]'s [`fmt` module][fmt] provides a subscriber for logging
  +[`tracing-subscriber`]'s [`fmt` module][fmt] provides a subscriber for logging
  traces with reasonable defaults. Additionally, `tracing-subscriber` is able to
  consume messages emitted by `log`-instrumented libraries and modules.
  