Description update for rcode.

[floydtree]

The current description of rcode field in the dns_activity class is not consistent with other sibling string fields in OCSF.

Currently it reads - The DNS server response code, as defined by the event source.

It should be updated to read - The server response code, normalized to the caption of the rcode_id value. In the case of 'Other', it is defined by the event source.

[floydtree]

Implemented in #489.