Fix overlays definition in example fleet.yaml in docs
In fleet 0.3.0 the list of yaml overlays must be in a yaml object instead of directly in the customization object (https://github.com/rancher/fleet/blob/786b72d2c0093f21f3a565efc86d083e7555dd1a/pkg/apis/fleet.cattle.io/v1alpha1/bundle.go#L175)
@@ -130,6 +130,7 @@ targetCustomizations:
# A file named ./overlays/myoverlay/subdir/resource_patch.yaml will patch the base file.
# A patch can in JSON Patch or JSON Merge format or a strategic merge patch for builtin
# Kubernetes types. Refer to "Raw YAML Resource Customization" below for more information.
+ yaml:
overlays:
- custom2
- custom3
