docs: networking: fix extra spaces in ethtool-netlink

Sphinx appears to get upset at extra spaces at the end of a literal:

Documentation/networking/ethtool-netlink.rst:1032: WARNING: Inline literal start-string without end-string.
Documentation/networking/ethtool-netlink.rst:1034: WARNING: Inline literal start-string without end-string.
Documentation/networking/ethtool-netlink.rst:1036: WARNING: Inline literal start-string without end-string.
Documentation/networking/ethtool-netlink.rst:1089: WARNING: Inline literal start-string without end-string.
Documentation/networking/ethtool-netlink.rst:1091: WARNING: Inline literal start-string without end-string.
Documentation/networking/ethtool-netlink.rst:1093: WARNING: Inline literal start-string without end-string.

Fixes: f2bc8ad3 ("net: ethtool: Allow PHY cable test TDR data to configured")
Fixes: a331172b ("net: ethtool: Add attributes for cable test TDR data")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>