man: avoid unportable use of utilities

echo -e is not portable. It can be replaced by printf %b (it works
only with the /bin/sh built-in echo, not /bin/echo anyway).
head -# is not portable, but head -n # is.

Replace these two things in three places total.

Signed-off-by: Mohamed Akram <mohd.akram@outlook.com>
Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1062