Documentation: Fix typo in freezer-subsystem.txt

Fix a typo in Documentation/cgroups/freezer-subsystem.txt.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Reviewed-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Acked-by: Randy Dunlap <rdunlap@xenotime.net>