ExternalDocumentRef.DocumentRefID does not perform required prefix processing because it is a string

[KAWAHARA-souta]

ExternalDocumentRef.DocumentRefID is a string.

spdx/v2/v2_3/document.go (L.19~23):

type ExternalDocumentRef struct {
	// DocumentRefID is the ID string defined in the start of the
	// reference. It should _not_ contain the "DocumentRef-" part
	// of the mandatory ID string.
	DocumentRefID string `json:"externalDocumentId"`

Although the comment states "It should not contain the 'DocumentRef-' part", this processing is not performed.
The type of this value should be common.DocElementID.Retry

[KAWAHARA-souta]

Resolved in #269