Fix spec CSS so that code font text does not mess up line spacing

[zimeon]

The current CSS for the specs, with boxes and padding around code font (backticked) text, results in an increased space below any line that include such text. This means that paragraphs were some lines include code font and some do not, render very messily. See, e.g. first para of https://iiif.io/api/presentation/3.0/#3-resource-properties

(Issue was noted in https://github.com/IIIF/api/issues/1550)