man(1): use gzcat for .gz files

POSIX zcat appends the .Z suffix to file arguments causing the command to fail
with .gz files.

Signed-off-by: Mohamed Akram <mohd.akram@outlook.com>

Reviewed by: markj
MFC after: 2 weeks
Pull Request: https://github.com/freebsd/freebsd-src/pull/789