netlink: specs: rt_addr: fix get multi command name

Command names should match C defines, codegens may depend on it.

Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Fixes: 4f280376 ("selftests/net: Add selftest for IPv4 RTM_GETMULTICAST support")
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20250403013706.2828322-3-kuba@kernel.org


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