Update basic-bonfires.json

Changes the current MDNlink to my suggested link on the issue https://github.com/FreeCodeCamp/FreeCodeCamp/issues/4459.

This PR closes https://github.com/FreeCodeCamp/FreeCodeCamp/issues/4459.

PS: I checked to make sure my suggested link is in the module bonfireMDNlinks.

seed/challenges/basic-bonfires.json

@@ -547,7 +547,7 @@
         "assert(mutation([\"hello\", \"neo\"]) === false, 'message: <code>mutation([\"hello\", \"neo\"])</code> should return false.');"
       ],
       "MDNlinks": [
-        "Array.indexOf()"
+        "String.indexOf()"
       ],
       "solutions": [
         "function mutation(arr) {\n  var hash = Object.create(null);\n  arr[0].toLowerCase().split('').forEach(function(c) {\n    hash[c] = true;\n  });\n  return !arr[1].toLowerCase().split('').filter(function(c) {\n    return !hash[c];\n  }).length;\n}\n\nmutation(['hello', 'hey']);\n"