qemu-storage-daemon: Fix help line for --export

Commit 5f479a8d renamed the 'device' option of --export into
'node-name', but forgot to update the help in qemu-storage-daemon.

Fixes: 5f479a8dc086bfa42c9f94e9ab69962f256e207f
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20200930133909.58820-1-kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

storage-daemon/qemu-storage-daemon.c

@@ -92,7 +92,7 @@ static void help(void)
 "  --chardev <options>    configure a character device backend\n"
 "                         (see the qemu(1) man page for possible options)\n"
 "\n"
-"  --export [type=]nbd,device=<node-name>,id=<id>,[,name=<export-name>]\n"
+"  --export [type=]nbd,id=<id>,node-name=<node-name>[,name=<export-name>]\n"
 "           [,writable=on|off][,bitmap=<name>]\n"
 "                         export the specified block node over NBD\n"
 "                         (requires --nbd-server)\n"