hello-world: set auto-approve to true (#1482)

The first core exercise on the track should be auto-approved.
This means that mentor approval isn't needed in order to move
forward when the exercise has been completed.

Currently any hello-world exercises are auto-approved in the
backend of v2 but this special handling might be removed later
which is why this also needs to be set in config.json.

config.json

       "difficulty": 1,
       "topics": [
         "strings"
-      ]
+      ],
+      "auto-approve": true
     },
     {
       "slug": "two-fer",