mitigations.7: uncomment reference to recently added section

A brief description of supervisor mode memory access protection was
added in 72ece341b427. Uncomment the existing reference to this section
in the table of contents.

Fixes: 72ece341b427 ("mitigations.7: mention supervisor mode memory...")
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 166ec3e86aa4a48bdf2a56ee065f0173bbb8f745)