git-debpush: Change comment heading

IMO "Principles of Operation" would be a comprehensive reference
detailing the model and functionality.  I think "Design Principles"
is the conventional name for semi-self-imposed choices/constraints.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>
Acked-by: Sean Whitton <spwhitton@spwhitton.name>