[editorial] Complete 19.6 example code (cut in the middle)

Fixes airbnb#2403

Author: kimhanui

README.md

@@ -2618,6 +2618,10 @@ Other Style Guides
 
     // good
     const leds = stage.selectAll('.led').data(data);
+    const svg = leds.enter().append('svg:svg');
+    svg.classed('led', true).attr(width', (radius + margin) * 2);
+    const g = svg.append('svg:g');
+    g.attr('transform', `translate(${radius + margin},${radius + margin})`).call(tron.led);
     ```
 
   <a name="whitespace--after-blocks"></a><a name="18.7"></a>