doc/netlink: Fix missing classic_netlink doc reference

Add missing cross-reference label for classic_netlink.

Fixes: 2db8abf0 ("doc/netlink: Document the netlink-raw schema extensions")
Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20230829085539.36354-1-donald.hunter@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>