docs: update link to #lima

docs: update link to #lima

Similar to the other channels, #lima is also moving to OFTC, so let's
update the links as appropriate.

While we're at it, fix the URI scheme to use a slash as a host/channel
separator, and drop the hash for maximum compatibility.

Reviewed-by: Erico Nunes <nunes.erico@gmail.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10917>