rustdoc: change `.src-line-numbers > span` to `.src-line-numbers > a`
Example: https://notriddle.com/notriddle-rustdoc-demos/line-anchors/test_dingus/fn.test.html
This allows people to treat them like real links, such as right-click to copy URL, and makes the line numbers in a scraped example work at all, when before this commit was added, they had the clickable pointer cursor but did not actually do anything when clicked.
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