update usage instructions about using a passphrase

[rustyrobin]

I propose a small addition to the passphrase section in the usage instructions:

wallet713> $ unlock -p (without yourpassphrase) allows hidden typing of passphrase. The standard instructions cause the passphrase to be displayed in plain text.

https://github.com/vault713/wallet713/blob/master/docs/usage.md#using-a-passphrase