message: Add missing plural
diff: @@ -70,7 +70,7 @@ Default and System Message Handlers
  -----------------------------------
  CAF has three system-level message types (``down_msg``, ``exit_msg``, and
  -``error``) that all actor should handle regardless of their current state.
  +``error``) that all actors should handle regardless of their current state.
  Consequently, event-based actors handle such messages in special-purpose message
  handlers. Additionally, event-based actors have a fallback handler for unmatched
  messages. Note that blocking actors have neither of those special-purpose
  