Update Anet A8 default values from my Anet A8 experience

Marlin/example_configurations/Anet/A8/Configuration.h

 #define EXTRUDERS 1
 
 // Generally expected filament diameter (1.75, 2.85, 3.0, ...). Used for Volumetric, Filament Width Sensor, etc.
-#define DEFAULT_NOMINAL_FILAMENT_DIA 3.0
+// The Anet A8 original extruder is designed for 1.75mm
+#define DEFAULT_NOMINAL_FILAMENT_DIA 1.75
 
 // For Cyclops or any "multi-extruder" that shares a single nozzle.
 //#define SINGLENOZZLE
 // It also enables the M302 command to set the minimum extrusion temperature
 // or to allow moving the extruder regardless of the hotend temperature.
 // *** IT IS HIGHLY RECOMMENDED TO LEAVE THIS OPTION ENABLED! ***
+// A temp of 160 guards against false tripping when the extruder fan kicks on.
 #define PREVENT_COLD_EXTRUSION
-#define EXTRUDE_MINTEMP 170
+#define EXTRUDE_MINTEMP 160
 
 // This option prevents a single extrusion longer than EXTRUDE_MAXLENGTH.
 // Note that for Bowden Extruders a too-small value here may prevent loading.
  * Override with M92
  *                                      X, Y, Z, E0 [, E1[, E2[, E3[, E4]]]]
  */
-#define DEFAULT_AXIS_STEPS_PER_UNIT   { 100, 100, 400, 95 }
+#define DEFAULT_AXIS_STEPS_PER_UNIT   { 100, 100, 400, 100 }
 
 /**
  * Default Max Feed Rate (mm/s)