Fix month field in a bibtex entry.

[bangerth]

bibtex does not understand 'month=5'. It needs to be 'month={may}'.

[tjhei]

Wait, didn't you just make the change may->5 in cite.html? Does month={5} work? Either way, please update cite.html too.

[bangerth]

The old way was to use month={5} (that's the before state of this patch), but looking at manual.pdf, this doesn't work.

I'll fix this in cite.html as well, but because that's a different root in the repo, I have to do this in a different PR.

[bangerth]

See #2307.