[guide] Fix 19.6 example code

Author: dancerphil

README.md

@@ -2619,7 +2619,7 @@ 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);
+    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);
     ```