doc/netlink: Fix missing classic_netlink doc reference

Add missing cross-reference label for classic_netlink.

Fixes: 2db8abf ("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>

Author: donaldh

Documentation/userspace-api/netlink/intro.rst

@@ -528,6 +528,8 @@ families may, however, require a larger buffer. 32kB buffer is recommended
 for most efficient handling of dumps (larger buffer fits more dumped
 objects and therefore fewer recvmsg() calls are needed).
 
+.. _classic_netlink:
+
 Classic Netlink
 ===============