Fix reference to images directory used with example config

The default dir might be `local-images` but on the example config [1] it is set to `images`.

[1]: https://github.com/ReshNesh/pixlserv/blob/da058be9c6b49f9a62900c047b63a3949791114a/config/example.yaml#L28
Merge pull request #5 from fgrehm/patch-1

Fix reference to images directory used with example config