docs(commit-style): update commit style guide

This patch updates commit-style.rst to reflect the rules enforced by
checkcommitmsg.py. A maximum width of 72 characters is now enforced for
commit messages.

Signed-off-by: Chuyue Luo <Chuyue.Luo@arm.com>
Change-Id: I88bbf345d53df050b1fa2680ff0ec1a120b34697
1 file changed