Add tabulation in example config

[vkatsuba]

Added tabulation in this block, by reason that here we expect the map but get boolean resulting in an error, so need change this

...
volumes:
  db:
  external: true

to this

...
volumes:
  db:
    external: true

[rehganavon]

I am not sure what has been changed or updated

[vkatsuba]

volumes:
  db:
  external: true <——

here is misssing tabulation and this configuration not working. For fix this issue I add tabulation in this code