Fix helptext for `rustup update`
Fix helptext for `rustup update`

The issue was raised in #528 that the help text for `rustup update` does not accurately reflect the behavior. This changes the help text to more accurately describe the behavior of the update subcommand.
theindigamer/rustup.rs,nodakai/rustup.rs,rust-lang/rustup,Boddlnagg/rustup.rs,inejge/rustup.rs,rust-lang/rustup,Boddlnagg/rustup.rs,inejge/rustup.rs,inejge/rustup.rs,Boddlnagg/rustup.rs,nodakai/rustup.rs,polonez/rustup.rs,nodakai/rustup.rs,Boddlnagg/rustup.rs,polonez/rustup.rs,theindigamer/rustup.rs,rust-lang/rustup,rust-lang/rustup,inejge/rustup.rs,nodakai/rustup.rs,theindigamer/rustup.rs,polonez/rustup.rs,inejge/rustup.rs,Boddlnagg/rustup.rs,nodakai/rustup.rs,polonez/rustup.rs,theindigamer/rustup.rs,theindigamer/rustup.rs,rust-lang/rustup,polonez/rustup.rs