docs: use consistent ext example

[skip ci]

Ref #1306

doc/sample-nodemon.md

@@ -22,7 +22,7 @@ Here is an example (of a contrived) `nodemon.json` file:
       "env": {
         "NODE_ENV": "development"
       },
-      "ext": "js json"
+      "ext": "js,json"
     }
 
 Note that the `ignore` used is nodemon's default ignore rule. The complete defaults can be seen here: [defaults.js](https://github.com/remy/nodemon/blob/master/lib/config/defaults.js).