man: RealTek -> Realtek

Realtek changed how it styled its name 25 or so years ago, but the old
style persisted in many places. These products use the new styling in
their datasheets.

Signed-off-by: ykla yklaxds@gmail.com
Sponsored by: Chinese FreeBSD Community
Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1901