image-handling.md: 'release' should be a string and not a list
image-handling.md: 'release' should be a string and not a list

  During 'lxc image import', yaml.Unmarshal() and is applied to the following struct:

  // ImageMetadata represents LXD image metadata
  type ImageMetadata struct {
	Architecture string                            `json:"architecture" yaml:"architecture"`
	CreationDate int64                             `json:"creation_date" yaml:"creation_date"`
	ExpiryDate   int64                             `json:"expiry_date" yaml:"expiry_date"`
	Properties   map[string]string                 `json:"properties" yaml:"properties"`
	Templates    map[string]*ImageMetadataTemplate `json:"templates" yaml:"templates"`
  }

  ImageMetadata.Properties is a map[string]string, and applying any property as a list will
  result in the following error:

  >> Error: Could not parse metadata.yaml: yaml: unmarshal errors:
  >> line 8: cannot unmarshal !!seq into string

Signed-off-by: fqbuild <a15bd1d84cff142ab83474f297c799e749a674ac@users.noreply.github.com>

hallyn/lxd,stgraber/lxd,Skarlso/lxd,lxc/lxd,stgraber/lxd,stgraber/lxd,atxwebs/lxd,hallyn/lxd,lxc/lxd,lxc/lxd,atxwebs/lxd,hallyn/lxd,Skarlso/lxd,Skarlso/lxd,stgraber/lxd,atxwebs/lxd,Skarlso/lxd,lxc/lxd,stgraber/lxd,stgraber/lxd,mjeanson/lxd,hallyn/lxd,hallyn/lxd,lxc/lxd,lxc/lxd,mjeanson/lxd,stgraber/lxd,mjeanson/lxd,lxc/lxd,atxwebs/lxd,hallyn/lxd