[docs] HTML Legend sample: add flex-shrink: 0 to color box (#11330)

This way the color box is prevented from shrinking in case of larger legends.

docs/samples/legend/html.md

@@ -61,6 +61,7 @@ const htmlLegendPlugin = {
       boxSpan.style.borderColor = item.strokeStyle;
       boxSpan.style.borderWidth = item.lineWidth + 'px';
       boxSpan.style.display = 'inline-block';
+      boxSpan.style.flexShrink = 0;
       boxSpan.style.height = '20px';
       boxSpan.style.marginRight = '10px';
       boxSpan.style.width = '20px';