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

Commit ce0e7b28fb ("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>

Documentation/filesystems/proc.txt

@@ -1255,9 +1255,9 @@ Various pieces   of  information about  kernel activity  are  available in the
 since the system first booted.  For a quick look, simply cat the file:
 
   > cat /proc/stat
-  cpu  2255 34 2290 22625563 6290 127 456 0 0
-  cpu0 1132 34 1441 11311718 3675 127 438 0 0
-  cpu1 1123 0 849 11313845 2614 0 18 0 0
+  cpu  2255 34 2290 22625563 6290 127 456 0 0 0
+  cpu0 1132 34 1441 11311718 3675 127 438 0 0 0
+  cpu1 1123 0 849 11313845 2614 0 18 0 0 0
   intr 114930548 113199788 3 0 5 263 0 4 [... lots more numbers ...]
   ctxt 1990473
   btime 1062191376