message: Fix typo in effect docs
diff: //! Some effects can be further classified into either announcements or requests, although these
  //! properties are not reflected in the type system.
  //!
  -//! **Announcements** are events emitted by components that are essentially "fire-and-forget"; the
  +//! **Announcements** are effects emitted by components that are essentially "fire-and-forget"; the
  //! component will never expect an answer for these and does not rely on them being handled. It is
  //! also conceivable that they are being cloned and dispatched to multiple components by the
  //! reactor.
  