docs: Add missing code block indicators to Slicers.md(Klipper3d#6161)

Signed-off-by: Paul Arthur <paul.arthur@flowerysong.com>

Author: flowerysong

docs/Slicers.md

@@ -103,7 +103,9 @@ START_PRINT BED_TEMP={material_bed_temperature_layer_0} EXTRUDER_TEMP={material_
 In slic3r derivatives such as PrusaSlicer and SuperSlicer, the
 following would be used:
 
+```
 START_PRINT EXTRUDER_TEMP=[first_layer_temperature] BED_TEMP=[first_layer_bed_temperature]
+```
 
 Also note that these slicers will insert their own heating codes when
 certain conditions are not met. In Cura, the existence of the