doc: mptcp: net.mptcp.pm_type is deprecated

doc: mptcp: net.mptcp.pm_type is deprecated

The net.mptcp.pm_type sysctl knob has been deprecated in v6.15,
net.mptcp.path_manager should be used instead.

Adapt the section about path managers to suggest using the new sysctl
knob instead of the deprecated one.

Fixes: 595c26d122d1 ("mptcp: sysctl: set path manager by name")
Cc: stable@vger.kernel.org
Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250908-net-mptcp-misc-fixes-6-17-rc5-v1-2-5f2168a66079@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>