Fix typo

@dysinger can you confirm that this is correct?

doc/yaml_configuration.md

@@ -106,7 +106,7 @@ Packages that come from the snapshot global database or are not affected.
 The image settings are used for the creation of container images using `stack container image`, e.g.
 ```yaml
 image:
- docker:
+ container:
  base: "fpco/stack-build"
  add:
  static: /data/static