doc(book/ffi): remove duplicate link reference `[libc]'

[vyp]

No description provided.

[rust-highfive]

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

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[vyp]

Sorry forgot to mention that the first identical [libc] link reference is at line 22.

[bluss]

Thank you

@bors r+ rollup

[bors]

📌 Commit fb5f3fa has been approved by bluss

[bors]

⌛ Testing commit fb5f3fa with merge 9e05951...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt