fix syntax test

Author: hisham waleed karam

src/plots/mapbox/layers.js

@@ -213,7 +213,7 @@ function convertSourceOpts(opts) {
     } else if (sourceType === 'raster') {
         field = 'tiles';
         sourceOpts['tileSize'] = 256;
-        for (let index = 0; index < source.length; index++) {
+        for (var index = 0; index < source.length; index++) {
             const url = source[index];
             source[index] = decodeURIComponent(url);