scripts: gitlint: Consistent case for Signed-off-by

The commit header as added by "git commit -s" is "Signed-off-by:",
and not any other casing. While it perhaps case-insensitive, use
canonical casing to not confuse (attentive) users.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
1 file changed