Rename demo area to match govuk-alerts

Relates to: https://github.com/alphagov/notifications-govuk-alerts/pull/152

I ran the "create-broadcast-areas-db.py" script to regenerate the
Sqlite DB. Existing alerts with the old naming still appear correctly,
and since we don't (yet) store this text in the DB, there's nothing
more to update.

app/broadcast_areas/source_files/Demo.geojson

@@ -5,7 +5,7 @@
       "type": "Feature",
       "properties": {
         "id": "sizewell",
-        "name": "Sizewell",
+        "name": "East Suffolk",
         "count_of_phones": 700
       },
       "geometry": {