Adapt help message for renamed binary

[pepa65]

Right now, the generic help message has these lines:

To send files, use `wormhole send <PATH>`.
To receive files, use `wormhole receive <CODE>`.

With the default binary name wormhole-rs this is not correct. If you rename (or softlink to) the binary, the USAGE line gets changed to for instance: rw [OPTIONS] <SUBCOMMAND>.
It would be nice if wormhole in the above lines also could automatically adapt to the binary name.

[felinira]

This just requires to change the help string here. IMO the information here is redundant, and we can just remove the last two lines from --help.

Then run cargo test and adjust the fallout in cli/tests/cmd.

Contributions welcome, this is a very low hanging fruit.