message: update tracing appender readme
diff: @@ -44,7 +44,7 @@ that can be used with _or_ without the non-blocking writer.
  Add the following to your `Cargo.toml`:
  ```toml
  -tracing-appender = "0.1"
  +tracing-appender = "0.2"
  ```
  This crate can be used in a few ways to record spans/events:
  