Minor docs change for `cargo test --help`
Minor docs change for `cargo test --help`

Per #10208, make `cargo test -- --help` more discoverable.

nipunn1313/cargo,rust-lang/cargo,rust-lang/cargo,eminence/cargo,nipunn1313/cargo,rustyhorde/cargo,rust-lang/cargo,eminence/cargo,rustyhorde/cargo,nipunn1313/cargo,eminence/cargo,rustyhorde/cargo,eminence/cargo,rust-lang/cargo,nipunn1313/cargo,rustyhorde/cargo