improve sample datasource.yaml

conf/provisioning/datasources/custom.yaml

@@ -1,11 +1,11 @@
 # # list of datasources that should be deleted from the database
-# delete_datasources:
+delete_datasources:
 #   - name: Graphite
 #     org_id: 1
 
 # # list of datasources to insert/update depending
 # # whats available in the datbase
-# datasources:
+datasources:
 #   # <string, required> name of the datasource. Required
 # - name: Graphite
 #   # <string, required> datasource type. Required