docs: Fix path paste-o for /sys/kernel/warn_count

Running "make htmldocs" shows that "/sys/kernel/oops_count" was
duplicated. This should have been "warn_count":

  Warning: /sys/kernel/oops_count is defined 2 times:
  ./Documentation/ABI/testing/sysfs-kernel-warn_count:0
  ./Documentation/ABI/testing/sysfs-kernel-oops_count:0

Fix the typo.

Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/linux-doc/202212110529.A3Qav8aR-lkp@intel.com
Fixes: 8b05aa26

 ("panic: Expose "warn_count" to sysfs")
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>