message: Tidy up message javadoc
diff: @@ -494,7 +494,7 @@ struct message : header<isRequest, Fields>
  The value of the Content-Length field will be unconditionally
  set to the specified number of octets.
  - @para n The number of octets to set for the Content-Length field.
  + @par n The number of octets to set for the Content-Length field.
  */
  void
  content_length(std::uint64_t n);
  