docs: ethtool: correct quotes

Quotes to backticks. All commands use backticks since the names
are constants.
Signed-off-by: NJakub Kicinski <kuba@kernel.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/networking/ethtool-netlink.rst

@@ -1433,7 +1433,7 @@ are netlink only.
   ``ETHTOOL_PHY_STUNABLE``            n/a
   ``ETHTOOL_GFECPARAM``               ``ETHTOOL_MSG_FEC_GET``
   ``ETHTOOL_SFECPARAM``               ``ETHTOOL_MSG_FEC_SET``
-  n/a                                 ''ETHTOOL_MSG_CABLE_TEST_ACT''
-  n/a                                 ''ETHTOOL_MSG_CABLE_TEST_TDR_ACT''
+  n/a                                 ``ETHTOOL_MSG_CABLE_TEST_ACT``
+  n/a                                 ``ETHTOOL_MSG_CABLE_TEST_TDR_ACT``
   n/a                                 ``ETHTOOL_MSG_TUNNEL_INFO_GET``
   =================================== =====================================