Update sample-config.json let's encrypt section

Reference Issues/PRs
Issues #3245 / #3296

what does this implement fix? Explain your changes.
setting skipChallengeVerification to true by default to save sometime when setting up a certificate

any other comments
I can see that this problem has been solved and a suggestion to make it easier
I think is to set "skipChallengeVerification": true by default.

sample-config.json

@@ -25,6 +25,7 @@
     "__comment__": "Requires NodeJS 8.x or better, Go to https://letsdebug.net/ first before trying Let's Encrypt.",
     "email": "myemail@mydomain.com",
     "names": "myserver.mydomain.com",
+    "skipChallengeVerification": true,
     "production": false
   }
 }