Adjust license header to refer to Invenio-pidstore

[fenekku]

Headers read:

# -*- coding: utf-8 -*-
#
# This file is part of Invenio.
# Copyright (C) 2015-2019 CERN.
#
# Invenio is free software; you can redistribute it and/or modify it
# under the terms of the MIT License; see LICENSE file for more details.

and it was suggested they read:

# -*- coding: utf-8 -*-
#
# This file is part of Invenio.
# Copyright (C) 2015-2019 CERN.
#
# Invenio-pidstore is free software; you can redistribute it and/or modify it
# under the terms of the MIT License; see LICENSE file for more details.

Someone can advise on the proper capitalization for Invenio-pidstore above.

[slint]

I think to make things easier on us, we've just labeled all the Invenio-* modules as part of "Invenio software" and to avoid having to be that explicit in the headers (see also the invenio-records-rest header).