fix typo in DKIM filter docs

[anarcat]

Surely you don't add dkin-ok tags (with an n)?

[GuillaumeSeren]

Hello @anarcat
Good catch please send more ! 💥

[codecov]

Codecov Report

Merging #318 (a7bcc94) into master (3b92bda) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #318   +/-   ##
=======================================
  Coverage   47.03%   47.03%           
=======================================
  Files          30       30           
  Lines        1080     1080           
=======================================
  Hits          508      508           
  Misses        572      572           

Flag	Coverage Δ
unittests	47.03% <ø> (ø)

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