sm: Fix certificate lookup in dirmngr cache.

* sm/call-dirmngr.c (get_cached_cert): Fix typo in LOOKUP command.

This bug has been here since the code was written. The commit message
for commit 2d0ca28a226773b9779a93c39bba9bace13232fe actually had the
warning that the code has not been tested. Now the funny thing is
that when the code was backported to the 2.0 branch just a few minutes
later that typo was fixed en passant in that branch.