replace makro artifact in references

Christoph Lienhard requested to merge fix_refs_in_doc into NIFTy_4

There was a left-over user-defined makro in the references section on the NIFTy homepage. This prevented copy-pasting the refs into my bib file because obviously this makro was not defined in my tex project. It should be fixed now. (I replaced the relevant line with one from a bibtex entry generated by scholar.google.com)

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