Fix reference to 'librlibc' in libcore docs

[wesleywiser]

Fixes #23059

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @brson (or someone else) soon.

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[brson]

@bors: rollup r+

Thanks!

[bors]

@bors r=brson 96b1f0c

[brson]

@bors: rollup r+

[bors]

@bors r=brson 96b1f0c

[brson]

@barosl: not sure why bors is striking out his comments above.

[eddyb]

@barosl or what's up with the time travels - is @homu modifying its own comments? (the hash on the first struck out comment above is the one of the commit following it)