update commit to agree with DCO (fixes #348)

[dabutvin]

from git help commit

-s, --signoff
           Add Signed-off-by line by the committer at the end of the commit
           log message. The meaning of a signoff depends on the project, but
           it typically certifies that committer has the rights to submit this
           work under the same license and agrees to a Developer Certificate
           of Origin (see https://developercertificate.org/ for more
           information).