Documentation/process: fix the canonical patch format description

There shouldn't be a blank line at the beginning, if there is no
optional in-body "From" line.  There must be a blank line between
the body of the explanation and the beginning of the S-o-b lines.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>