Update misleading sample comment (#9865)

Author: kurkle

docs/samples/scale-options/ticks.md

@@ -76,7 +76,7 @@ const config = {
         ticks: {
           // For a category axis, the val is the index so the lookup via getLabelForValue is needed
           callback: function(val, index) {
-            // Hide the label of every 2nd dataset
+            // Hide every 2nd tick label
             return index % 2 === 0 ? this.getLabelForValue(val) : '';
           },
           color: 'red',