getrusage: minor typo (runivcsw -> ru_nivcsw)

Hello,
The `getrusage` example listing contains a small typo: the underscore in `ru_nivcsw` was missing.
See `man 2 getrusage` to confirm.

Thank you for this wonderful book!
I tried sending the patch using `git send-email` first but I couldn't get it to work. I apologise in advance in case you receive two emails.

Signed-off-by: Han-Miru Kim
<hanmiru.kim@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
1 file changed