message: docs: fix typo surrounding 'log-always' usage
diff: //! is intended for use in libraries whose users may be using either `tracing`
  //! or `log`.
  //! * `log-always`: Emit `log` records from all `tracing` spans and events, even
  -//! a `tracing` subscriber has been set. This should be set only by
  +//! if a `tracing` subscriber has been set. This should be set only by
  //! applications which intend to collect traces and logs separately; if an
  //! adapter is used to convert `log` records into `tracing` events, this will
  //! cause duplicate events to occur.
  