dns/prometheus-dnssec-exporter: fix distinfo

Spell DISTVERSION correctly in distinfo. Upstream does not use version
numbers. I originally used the date I created the port as DISTVERSION.
I forgot to update distinfo after correcting that to be the date of the
upstream commit per convention.

Pointy hat to: philip

(cherry picked from commit 0d78aa54ad645a88a1a6f297ecc84e6543245572)