Ender 3 Simple Bed Level Fix

Reddit user [hoges](https://www.reddit.com/user/hoges) posted an interesting thread in the [Ender 3](http://ender3.reddit.com) subreddit about a fix for an issue that he'd discovered in troubleshooting difficulties with bed leveling.

> I discovered a couple of days ago that the eccentric nuts and the spacers which are used on the XY carriage are ~0.5mm different in height. That means that the carriage will be on an angle from the outset and need to be compensated with the springs.
>
> This can lead to needing one spring fully compressed while on the other side the spring is at near max expansion and providing little stability.
>
> Though 0.5mm doesn't sound like much, the difference is at the centre of the carriage so the effect of this is magnified towards the edges of the X-axis upwards of 5mm.

All this model consists of is a 0.5mm spacer that fits over the existing spacer on one side of the bed. I've tried to illustrate this in the attached photos, as some users request