userguide: update tls eve-log fields 'not_before' and 'not_after'
It was reported that our documentation mentioned the tls fields 'not_before' and 'not_after'
as possible custom fields for the tls events in our eve-log, whereas in fact the fields
themselves are written as 'notbefore' and 'notafter', which led to confusion for folks trying
to follow the documentation to parse our logs.
Our documentation: https://suricata.readthedocs.io/en/latest/output/eve/eve-json-format.html#id10
Example check for our eve-log: https://github.com/OISF/suricata-verify/blob/master/tests/bug-2646-01/test.yaml#L20
Update our documentation to reflect what is seen in our logs.