Incorrect instructions for SASL setup on WeeChat 4.0+

[emanuele6]

The instructions at https://libera.chat/guides/weechat for "Configuring SASL for WeeChat" say to use:

/set irc.server.libera.sasl_mechanism PLAIN

But, in WeeChat 4.0 or later, that will actually result in an error:

Failed to set option "irc.server.libera.sasl_mechanism"

PLAIN must be written in lowercase:

/set irc.server.libera.sasl_mechanism plain