docs RTD theme: code-block with line nos - lines and line numbers don't line up.

docs RTD theme: code-block with line nos - lines and line numbers don't line up.

In a code-block with line numbers (option :lineno:) there is a
misalignment of the rendered source code lines on the right side and the
line numbers on the left side.

  https://github.com/rtfd/sphinx_rtd_theme/issues/419

Since this issue is reported to the RTD theme project, it might be fixed
in the future (take this as a interim solution).

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>