Book: Fix link anchor in Syntax Index

[floftar]

See: #30397

r? @steveklabnik

[rust-highfive]

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

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

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[steveklabnik]

@bors: r+ rollup

Thank you so much!

[tshepang]

Merge commit should be removed

[floftar]

@tshepang Thank you for pointing out my mistake.

[steveklabnik]

I dunno why bors didn't see this

@bors: r+ rollup

[bors]

📌 Commit b9005dd has been approved by steveklabnik

[bors]

⌛ Testing commit b9005dd with merge da4a21e...

[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

[tshepang]

@steveklabnik An additional commit was made after approval