net: ena: Fix rst format issues in readme

This patch fixes a warning in the ena documentation
file identified by the kernel automatic tools.
The patch also adds a missing newline between
sections.

Signed-off-by: David Arinzon <darinzon@amazon.com>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202306171804.U7E92zoE-lkp@intel.com/
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Link: https://lore.kernel.org/r/20230620133544.32584-1-darinzon@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>