criu --help: fix -D description

-D is used for dump and restore, i.e. to read and write image files,
depending on the command, so let's put a read/write-neutral description.
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

crtools.c

@@ -380,7 +380,7 @@ usage:
 "  -d|--restore-detached detach after restore\n"
 "  -s|--leave-stopped    leave tasks in stopped state after checkpoint\n"
 "  -R|--leave-running    leave tasks in running state after checkpoint\n"
-"  -D|--images-dir DIR   directory where to put images to\n"
+"  -D|--images-dir DIR   directory for image files\n"
 "     --pidfile FILE     write a pid of a root task to this file\n"
 "\n"
 "* Special resources support:\n"