[Improvement] Change unknown command message to tell players to type "/help", not "help". Fixes BUKKIT-4614

## The Issue

The unknown command message tells players to type "help", but typing "help" does nothing.

## Justification for this PR

Clarity for users.

## PR Breakdown

When sending the message, "help" is used in the case of the console, and "/help" is used otherwise.

## JIRA Ticket

BUKKIT-4614 - https://bukkit.atlassian.net/browse/BUKKIT-4614