HPGL plotter fix incorrect scaling factor

A 0.4% scale error in hpgl plots was previously existing since a long time.

Fixes: lp:1772828
https://bugs.launchpad.net/kicad/+bug/1772828

common/plotters/HPGL_plotter.cpp

@@ -210,8 +210,9 @@
 // EP;   draws the polygon outline. It usually gives a better look to the filled polygon
 static const char hpgl_end_polygon_cmd[] = "PM 2; FP; EP;\n";
 
-// HPGL scale factor (1 PLU = 1/40mm = 25 micrometers)
-static const double PLUsPERDECIMIL = 0.102041;
+// HPGL scale factor (1 Plotter Logical Unit = 1/40mm = 25 micrometers)
+// PLUsPERDECIMIL = (25.4 / 10000) / 0.025
+static const double PLUsPERDECIMIL = 0.1016;
 
 HPGL_PLOTTER::HPGL_PLOTTER()
 {