Improve usage message for /effect

**The Issue:**
When the usage message is sent to the player, It only sends the message, without specifying "Usage: "

**Justification for this PR:**
This PR helps with command usage & helps to provide consistency between commands.

**JIRA Ticket:**
https://bukkit.atlassian.net/browse/BUKKIT-5375

**Testing Material**
Compiled changes from local fork and tested.

Associated Bukkit PR: https://github.com/Bukkit/Bukkit/pull/1027