edit milling

UrumbotXY/README.md

@@ -52,7 +52,7 @@ Movie:
 
 <img src=video/milling_movie.mp4 width=100%><br>
 
-Closeup of traces:  Using 1/64" bit and 0.8 mm separation of paths, at a 45 degree angle, the calculated trace width is 0.3 mm.  Looks pretty close to this.  
+Closeup of traces:  Using 1/64" bit and 0.8 mm horizontal separation of paths, at a 45 degree angle, the calculated trace width is 0.3 mm.  Looks pretty close to this.  
 
 <img src=img/traces.jpg width=50%><br>