fix test on es6 export default (#15627)

challenges/02-javascript-algorithms-and-data-structures/es6.json

@@ -832,16 +832,16 @@
         "In the <code>export</code> lesson, you learned about the syntax referred to as a <dfn>named export</dfn>. This allowed you to make multiple functions and variables available for use in other files.",
         "There is another <code>export</code> syntax you need to know, known as <dfn>export default</dfn>. Usually you will use this syntax if only one value is being exported from a file. It is also used to create a fallback value for a file or module.",
         "Here is a quick example of <code>export default</code>:",
-        "<blockquote>export default const add = (x,y) => {<br>  return x + y;<br>}</blockquote>",
+        "<blockquote>export default function add(x,y) {<br>  return x + y;<br>}</blockquote>",
-        "There is a one major feature of <code>export default</code> you must never forget - since it is used to declare a fallback value for a module or file, you can only have one value be a default export in each module or file.",
+        "Note: Since <code>export default</code> is used to declare a fallback value for a module or file, you can only have one value be a default export in each module or file. Additionally, you cannot use <code>export default</code> with <code>var</code>, <code>let</code>, or <code>const</code>",
         "<hr>",
         "The following function should be the fallback value for the module. Please add the necessary code to do so."
       ],
       "challengeSeed": [
-        "const subtract = (x,y) => {return x - y;}"
+        "function subtract(x,y) {return x - y;}"
       ],
       "tests": [
-        "assert(code.match(/export\\s+default\\s+const\\s+subtract\\s+=\\s+\\(x,y\\)\\s+=>\\s+{return\\s+x\\s-\\s+y;}/ig))"
+        "assert(code.match(/export\\s+default\\s+function\\s+subtract\\(x,y\\)\\s+{return\\s+x\\s-\\s+y;}/ig))"
       ],
       "type": "waypoint",
       "releasedOn": "Feb 17, 2017",