Remove "@api private" tag from Flow#write documentation

Any public method is part of the public interface of the class and thus
needs to be equally well supported. Only because methods are not usually
called by end-users, we still need to properly document, test, and check
them.

lib/rackstash/flow.rb

@@ -206,8 +206,6 @@ module Rackstash
     # 3. Finally, the encoded event will be passed to the {#adapter} to be send
     #    to the actual log target, e.g. a file or an external log receiver.
     #
-    # @api private
-    #
     # @param event [Hash] an event hash
     # @return [Boolean] `true` if the event was written to the adapter, `false`
     #   otherwise