Improve text flow in 'rustup completions -h'
Improve text flow in 'rustup completions -h'

Sentences are now properly terminated with a full stop and the shell
command lines are now mostly introduced with a ':' for consistency.

Boddlnagg/rustup.rs,nodakai/rustup.rs,Boddlnagg/rustup.rs,rust-lang/rustup,inejge/rustup.rs,Boddlnagg/rustup.rs,rust-lang/rustup,nodakai/rustup.rs,nodakai/rustup.rs,rust-lang/rustup,inejge/rustup.rs,inejge/rustup.rs,rust-lang/rustup,inejge/rustup.rs,rust-lang/rustup,Boddlnagg/rustup.rs,nodakai/rustup.rs,nodakai/rustup.rs,inejge/rustup.rs,Boddlnagg/rustup.rs