doc: fix description of L2TPV2 flow item

[ upstream commit a7d0dd8693417fbecb0debe7b4aec96dbafed51a ]

The flow item structure includes the protocol definition
from the directory lib/net/, so it is reflected in the guide.

Section title underlining is also fixed around.

Fixes: 3a929df ("ethdev: support L2TPv2 and PPP procotol")

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Ferruh Yigit <ferruh.yigit@amd.com>
Acked-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Acked-by: Chengwen Feng <fengchengwen@huawei.com>

doc/guides/prog_guide/rte_flow.rst

@@ -1610,22 +1610,15 @@ rte_flow_flex_item_create() routine.
   value and mask.
 
 Item: ``L2TPV2``
-^^^^^^^^^^^^^^^^^^^
+^^^^^^^^^^^^^^^^
 
 Matches a L2TPv2 header.
 
-- ``flags_version``: flags(12b), version(4b).
-- ``length``: total length of the message.
-- ``tunnel_id``: identifier for the control connection.
-- ``session_id``: identifier for a session within a tunnel.
-- ``ns``: sequence number for this date or control message.
-- ``nr``: sequence number expected in the next control message to be received.
-- ``offset_size``: offset of payload data.
-- ``offset_padding``: offset padding, variable length.
+- ``hdr``:  header definition (``rte_l2tpv2.h``).
 - Default ``mask`` matches flags_version only.
 
 Item: ``PPP``
-^^^^^^^^^^^^^^^^^^^
+^^^^^^^^^^^^^
 
 Matches a PPP header.