Link to PWA Checklist (#1734)

* Link to PWA Checklist

removed the extra variable and added it to description

correction

* Update default.json

* Update sample.json

* removed handlebar each reference

lighthouse-cli/test/fixtures/sample.json

@@ -447,7 +447,7 @@
   "aggregations": [
     {
       "name": "Progressive Web App",
-      "description": "These audits validate the aspects of a Progressive Web App.",
+      "description": "These audits validate the aspects of a Progressive Web App. They are a subset of the [PWA Checklist](https://developers.google.com/web/progressive-web-apps/checklist).",
       "scored": true,
       "total": 0.40625,
       "categorizable": true,

lighthouse-core/config/default.json

@@ -103,7 +103,7 @@
 
   "aggregations": [{
     "name": "Progressive Web App",
-    "description": "These audits validate the aspects of a Progressive Web App.",
+    "description": "These audits validate the aspects of a Progressive Web App. They are a subset of the [PWA Checklist](https://developers.google.com/web/progressive-web-apps/checklist).",
     "scored": true,
     "categorizable": true,
     "items": [{