fix bib entry for the manual

[tjhei]

now it is the same as https://aspect.dealii.org/publications.html

[bangerth]

OK if you take the entry out of the release task list again that requires updating the bibtex entry. (Or should we leave that in, and just make sure that the year is appropriate?)

[tjhei]

(Or should we leave that in, and just make sure that the year is appropriate?)

Yes, we should do that.

[bangerth]

OK to merge then.