exit.3: add the comma after an empty space

exit(3) man page shows cxa_atexit(3,) instead of cxa_atexit(3), in a
particular section. It seems the comma gets inside the parenthesis and
with an extra space, it can be viewed as expected.

Signed-off-by: rilysh <nightquick@proton.me>
Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1006