Fix bare links in docs against Rust 1.53

[jdisanti]

Rust 1.53 added a lint for bare URLs: rust-lang/rust#81764

This PR fixes doc generation so that docs continue to build with the latest stable compiler, but does not attempt to be the long-term documentation normalization strategy.

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[rcoh]

would be nice to have a generated code diff to audit

[jdisanti]

On second thought, there isn't really a good way to fix this without full parsing of the documentation. There are cases in the documentation such as this that should not get transformed:

<code>curl
http://someurl</code>

Or

<code>[ "CMD-SHELL", "curl -f http://localhost/ || exit 1" ]</code>

I'll table this and start on the long-term approach.