config: DOI/arXiv database search prefixes

* Updates DOI/arXiv database search prefixes when used in CrossRef and arXiv Flask extensions.
@jmartinm my bad, it was changed in [here](https://github.com/inveniosoftware/invenio/blob/pu/invenio/ext/crossref/__init__.py#L137) and I didn't notice, but now it's correct. ;)