Description of problem:

The comment line generated with the DKIM record shows an 'i=' (identifier) tag, while this should be the 's=' (selector) tag.

Version-Release number of selected component (if applicable):

2.12.0

How reproducible:

set up a DKIM key pair in the configuration and run the command:
  amavisd -c /etc/amavisd/amavisd.conf showkeys

the first line of a/each DKIM record is a comment line, now saying something like this:
  ; key#27 4096 bits, i=dkim20191228, d=example.com, 
      /etc/amavisd/example.com.dkim20191228.pem
this should be:
  ; key#27 4096 bits, s=dkim20191228, d=example.com, 
      /etc/amavisd/example.com.dkim20191228.pem

Additional info:

Since this is only a comment line providing info on the actual record that follows, there's no harm done, other than suggesting that this is the identifier rather than the selector tag.