[PATCH -perfbook] getrusage: minor typo (runivcsw -> ru_nivcsw)

[Hanmiru Kim]

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....@gmail.com>
---
diff --git a/debugging/debugging.tex b/debugging/debugging.tex
index f285ebc1..ccc33ef9 100644
--- a/debugging/debugging.tex
+++ b/debugging/debugging.tex
@@ -2609,7 +2609,7 @@ int runtest(void)
                abort();
        }
        return (ru1.ru_nvcsw == ru2.ru_nvcsw &&
-               ru1.runivcsw == ru2.runivcsw);
+               ru1.ru_nivcsw == ru2.ru_nivcsw);
 }
 \end{VerbatimL}
 \end{fcvlabel}