fix(example): fix errors in example file

[GMishx]

Fix the following error/warnings in example script for creating documents from scratch.

WARNING:root:did not find the referenced spdx_id "SPDXRef-Document" in the SPDX document
WARNING:root:ValidationContext(spdx_id=None, parent_id=None, element_type=<SpdxElementType.RELATIONSHIP: 13>, full_element=Relationship(spdx_element_id='SPDXRef-Document', relationship_type=<RelationshipType.DESCRIBES: 14>, related_spdx_element_id='SPDXRef-Package', comment=None))
WARNING:root:did not find the referenced spdx_id "SPDXRef-File1" in the SPDX document
WARNING:root:ValidationContext(spdx_id=None, parent_id=None, element_type=<SpdxElementType.RELATIONSHIP: 13>, full_element=Relationship(spdx_element_id='SPDXRef-Package', relationship_type=<RelationshipType.CONTAINS: 6>, related_spdx_element_id='SPDXRef-File1', comment=None))
WARNING:root:did not find the referenced spdx_id "SPDXRef-File2" in the SPDX document
WARNING:root:ValidationContext(spdx_id=None, parent_id=None, element_type=<SpdxElementType.RELATIONSHIP: 13>, full_element=Relationship(spdx_element_id='SPDXRef-Package', relationship_type=<RelationshipType.CONTAINS: 6>, related_spdx_element_id='SPDXRef-File2', comment=None))

Add the files to package and update the SPDX id describing relationship between document and package.

[armintaenzertng]

Thanks for the fix, very much appreciated! :)