Documentation: update epoll sysctl text

max_user_instances was removed in this commit:

   commit 9df04e1f


   Author: Davide Libenzi <davidel@xmailserver.org>
   Date:   Thu Jan 29 14:25:26 2009 -0800

    epoll: drop max_user_instances and rely only on max_user_watches

but the documentation entry was not removed.

Cc: Davide Libenzi <davidel@xmailserver.org>
Signed-off-by: Lucian Adrian Grijincu <lucian.grijincu@gmail.com>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>