rustdoc: make line number CSS for doc comment and scraped the same
![image](https://user-images.githubusercontent.com/
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Discussed in
https://rust-lang.zulipchat.com/#narrow/stream/266220-rustdoc/topic/Line.20number.20styling
Preview: http://notriddle.com/notriddle-rustdoc-demos/source-lines/test_dingus/fn.test.html