netlink: specs: mptcp: replace underscores with dashes in names

netlink: specs: mptcp: replace underscores with dashes in names

We're trying to add a strict regexp for the name format in the spec.
Underscores will not be allowed, dashes should be used instead.
This makes no difference to C (codegen, if used, replaces special
chars in names) but it gives more uniform naming in Python.

Fixes: bc8aeb2045e2 ("Documentation: netlink: add a YAML spec for mptcp")
Reviewed-by: Davide Caratti <dcaratti@redhat.com>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250624211002.3475021-8-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>