The following gives an example of the preferred way for citing entries in the AFP:
M. Jaskelioff and S. Merz, Proving the Correctness of Disk Paxos. Archive of Formal Proofs, June 2005, http://isa-afp.org/entries/DiskPaxos.html, Formal proof development.
The bibtext entry for this would be:
@article{Jaskelioff-Merz-AFP05,
author = {Mauro Jaskelioff and Stephan Merz},
title = {Proving the Correctness of Disk Paxos},
journal = {Archive of Formal Proofs},
month = Jun,
year = 2005,
note = {\url{http://isa-afp.org/entries/DiskPaxos.html}, Formal proof development},
ISSN = {2150-914x}
}