Update phosaa14SB reference

[IAlibay]

Very minor thing.

The phosaa14SB paper has now been released: https://pubs.acs.org/doi/full/10.1021/acs.jctc.4c00732

It might be good to update the reference accordingly (probably in ~ a month when the volume number etc.. gets released): https://github.com/openmm/openmmforcefields/blob/main/openmmforcefields/ffxml/amber/phosaa14SB.xml#L5

[epretti]

As of #368 the reference is included.