addresses issue in #353 label obsuring inner div

[finanalyst]

• in a code box, the 'language' label was obscuring the contents of the code box it labelled
• code box contents were overflowing to right
• this patch does not change problem in #396

[finanalyst]

@coke this is a CSS-only change.

[coke]

353 (so we have a link to the ticket)