jail.conf.5: Fix mandoc typos

MFC after: 3 days
Reviewed by: ziaee
Signed-off-by: Rob Nichols <robert.nichols@dialwave.com>
Closes: https://github.com/freebsd/freebsd-src/pull/1928

(cherry picked from commit d1d88b6e8c31b1e472d66471ff1e666e5310709e)