docs: dev-tools: coccinelle: underlines

Underline lengths should match the lengths of headings to avoid build
warnings with Sphinx.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Acked-by: Julia Lawall <julia.lawall@inria.fr>
Link: https://lore.kernel.org/r/20200605030405.6479-1-xypron.glpk@gmx.de


Signed-off-by: Jonathan Corbet <corbet@lwn.net>