DOC: ring: refer to newer RFC5424

DOC: ring: refer to newer RFC5424

In the ring configuration example we refer to RFC3164 - the original BSD
syslog protocol without support for structured data (SDATA).

Let's refer to RFC5424 instead so SDATA is by default forwarded if
someone copy & pastes from the documentation:

https://discourse.haproxy.org/t/structured-data-lost-when-forwarding-logs-voa-syslog-forwarding-feature/11741/5

Should be backported to 2.6.

(cherry picked from commit 5f9ce99c799eaa8f58518201b58c32d970b10cdf)
Signed-off-by: Willy Tarreau <w@1wt.eu>