Tiny improvements for z-probe image

arekm

I would like to suggest tiny improvements for

https://www.repetier.com/firmware/dev/images/extruderandzprobeoffsets.png </div><div>image:

- explictly mark (0,0) point (including coordinates) in the middle between extruders (people don't notice that zero is exactly in middle between extruders)

- offset probe a bit into left (for example) in X axis, so it will have negative X value (people are not sure when values should be positive/negative for X offset)

Maybe even second image with just one extruder and z probe (so (0,0) will be in middle of extruder).

These suggestions come from discussion on one 3d printing forum where people are asking and are still not sure with current images.

Repetier

I always thought it was crystal clear with the axis direction and origin drawn between the extruders:-)

Will improve it for dev version.