Fix link to LLVM libcalls-sync on custom targets page

[calvinkosmatka]

Found a typo'd link on the custom targets page

[rust-highfive]

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

Please see the contribution instructions for more information.

[bors[bot]]

Build failed:

• build (1.51.0)

[adamgreig]

bors retry

[bors[bot]]

Build succeeded:

• build (1.58.1)
• build (stable)