doc: Add guest_nice column to example output of `cat /proc/stat'

Commit ce0e7b28 ("sched, cpuacct: Fix niced guest time
accounting") added the guest_nice column to /proc/stat, but the example
output of `cat /proc/stat' in Documentation/filesystems/proc.txt wasn't
updated accordingly. Do so now.

Cc: Ryota Ozaki <ozaki.ryota@gmail.com>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>