ANET A8 Rear Bed Level Sensor Holder

The original support by TNS hits the frame of my Anet A8, so I edited it removing a small part on the right side to prevent collision.

These are the parameters I set in Marlin configuration:

`#define X_PROBE_OFFSET_FROM_EXTRUDER 27`
`#define Y_PROBE_OFFSET_FROM_EXTRUDER 57`
`#define Z_PROBE_OFFSET_FROM_EXTRUDER 0`

And, optionally, to be universally usable:

`#define FRONT_PROBE_BED_POSITION (Y_MIN_POS + Y_PROBE_OFFSET_FROM_EXTRUDER + 1)`