_content: fix markdown styling with dark theme under /lib

[xieyuschen]

I found the page https://go.dev/lib/godoc/analysis/help isn't rendered properly under the dark mode, as I mentioned in golang/go#66659.

I fixed it with the same idea in fix #269.

Regards

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Patch Set 1:

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[xieyuschen]

closed this PR as the right direction is deleted the godoc so the page here will be deleted as well.

golang/go#59056