tty.4: see also: cross-reference pts(4)

pts(4) already refers to tty(4).

Add a cross-reference.

Reviewed by: kevans
Approved by: kevans (mentor)
Pull Request: https://github.com/freebsd/freebsd-src/pull/1617

(cherry picked from commit 6f4c4cfe4b138297957d80c0887f99a83d9facf7)