Fix typo in custom allocator docs.

[solson]

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.

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[steveklabnik]

@bors: r+ rollup

[steveklabnik]

Thanks a ton!

[solson]

The build error is spurious.

[steveklabnik]

@bors: r+ rollup

[bors]

📌 Commit 0d3d6e4 has been approved by steveklabnik

[bors]

⌛ Testing commit 0d3d6e4 with merge ca998fc...

[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