exclude H3 from connection type arguments list

[Devel08]

H3 isn't an implemented connection type, thus indeed panics, this PR removes it from the arguments list, so that the user cannot cause a panic, possible values will be h1-clear-text, h1 and h2.

$ cargo run -q saturate -t
error: a value is required for '--conn-type <CONN_TYPE>' but none was supplied
  [possible values: h1-clear-text, h1, h2]

For more information, try '--help

ran cargo fmt and cargo clippy. Closes #51