"Non-standard MQTT options" documents message_size_limit instead of max_message_size

[Jesse Friedman]

The page at https://docs.vernemq.com/configuration/nonstandard documents the message_size_limit option, but that option is deprecated:

[warning] message_size_limit is deprecated and has no effect, use max_message_size instead

[Lars Hesel Christensen]

Thanks for pointing it out. I've just pushed a fix!