single-command help shows `<prefix>` for bot with no prefix

[SIGSTACKFAULT]

I don't want my bot to have a prefix; I want to invoke commands solely by pinging it.

Annoyingly, poise's built in help function shows the prefix as <prefix>

it would be acceptable if it was just nothing or @bot_name (well, preferably configurable, but i'll take either)

It happens at help.rs line 158

general help doesn't have this problem: