use the ~ operator instead of = to fix bad <code> block indentation

@jina This fixes the strange indentation of the code block output. Something that I am noticing about the Rouge::Themes::Base16::Solarized business that is formatting/styling code blocks... is that there is pretty poor contrast on some of the elements. Particularly the orange and green are really low contrast and then the dark for the .err class generated, that has a low text/background contrast... but a really jarring high contrast against the main pre.highlight background. Any objections to specifying style overrides for those cases to improve the visibility and general feeling of this stuff? Is there a better way to access the styles in the magic provided by the library we are using for code formatting/styling?

cc/ @bpainter