Fix heading fragments on "Information security"

I'm not totally convinced that these fragment links are worth it,
because they break fairly frequently. But if they're going to be
there they should at least be correct.
Remove unnecessary additional link text

I fixed once instance of the typo "Guidence", but neglected this one.

After reading it, there's no need to have the text repeated here.
Merge pull request #405 from alphagov/bugfix-infosec-heading-links

Fix heading fragments on "Information security"