message: subscriber: fix outdated documentation
  The filter feature has already been removed.
diff: //!
  //! - `env-filter`: Enables the [`EnvFilter`] type, which implements filtering
  //! similar to the [`env_logger` crate]. Enabled by default.
  -//! - `filter`: Alias for `env-filter`. This feature flag was renamed in version
  -//! 0.1.2, and will be removed in version 0.2.
  //! - `fmt`: Enables the [`fmt`] module, which provides a subscriber
  //! implementation for printing formatted representations of trace events.
  //! Enabled by default.
  