Fix comment typo

src/axis.js

@@ -396,7 +396,7 @@ c3_chart_internal_fn.tuneAxis = function(sync, callback){
 
         // $$.getAxisData should be injected by ed3
         // If it's not, it's either that c3 is running in test enviroment
-        // or some error accured
+        // or some error occurred
         if($$.getAxisData){
             if($$.config.is_xy){
                 sizesX = $$.getAbscissa($$.config.ed3Type, minmax.minX, minmax.maxX);