docs: security: fix section hyperlink

The reStructuredText syntax is wrong here; not sure how it was
intended but we can just use the section header as an implicit
hyperlink target, with a single "outward" underscore.

Signed-off-by: Brendan Jackman <bhenryj0117@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>