[header] Replace CARGO with crates.io
[header] Replace CARGO with crates.io

As Cargo documentation is being moved to <doc.rust-lang.org>, and to
emphasis the difference between `crates.io` and `cargo`, we better not
title the website as *CARGO* anymore.

This is a quick fix for this, until we redirect `docs.crates.io`
completely and fix the menus and links here.

See <https://github.com/rust-lang/crates.io/issues/1031>

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