fix description for .tz set command

[fadedmax]

The previous reminisce of when the bot still stored a offset is still in the command, and can be distracting, this should be changed to something more accurate of the current bot. i.e right now it's Sets and stores your UTC offset into the bot. maybe it should be something different as .tz set UTC-5 doesn't work. [also using this as a demo for the issue command i know this doesn't have to be a issue]