book: Fix typo

[salty-horse]

Small typo. vectors -> vector.

[rust-highfive]

r? @brson

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[apasel422]

@bors: r+ rollup

Thanks!

[bors]

📌 Commit d6d949a has been approved by apasel422

[bors]

⌛ Testing commit d6d949a with merge 72d2b24...

[bors]

💔 Test failed - auto-linux-64-nopt-t

[apasel422]

@bors: retry

[bors]

⌛ Testing commit d6d949a with merge 2ced149...

[bors]

💔 Test failed - auto-linux-32-opt

[apasel422]

@bors: retry

[bors]

⚡ Previous build results for auto-linux-32-nopt-t, 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-64-opt, auto-win-msvc-32-opt are reusable. Rebuilding only auto-linux-32-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-msvc-64-opt...

[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