change G26 default nozzle size to 0.4, consistent with instructions (#6506)

Marlin/G26_Mesh_Validation_Tool.cpp

 
   #define EXTRUSION_MULTIPLIER 1.0
   #define RETRACTION_MULTIPLIER 1.0
-  #define NOZZLE 0.3
+  #define NOZZLE 0.4
   #define FILAMENT 1.75
   #define LAYER_HEIGHT 0.2
   #define PRIME_LENGTH 10.0