docs say 0 is erase color

README.md

@@ -60,7 +60,7 @@ Colors 1-15 are used in the color index, documented here:
 With giza, it renders more like this:
 
 ```
-0   Black
+0   Erase ?
 1   Black
 2   Orange
 3   Forest green

skymap.c

@@ -1731,7 +1731,7 @@ void skymap_plotsatellite(char *filename,int satno,double mjd0,double dt)
 	    break;
 	  } else {
 	    if (s.salt<0.0)
-	      cpgsci(7);
+	      cpgsci(128);
 	    else
 	      cpgsci(8);
 	  }