ntp.conf.5: Fix typos

"certian" --> "certain"
"compatability" --> "compatibility"
"noticable" --> "noticeable"
"limite" --> "limit"
"acutal" --> "actual"
"ba" --> "be"

MFC after: 1 week
Pull Request: https://github.com/freebsd/freebsd-src/pull/550
Signed-off-by: Elyes HAOUAS <ehaouas@noos.fr>