net/tcp/illinois: replace broken algorithm reference link
The link to the pdf containing the algorithm description is now a
dead link; it seems http://www.ifp.illinois.edu/~srikant/ has been
moved to https://sites.google.com/a/illinois.edu/srikant/ and none of
the original papers can be found there...
I have replaced it with the only working copy I was able to find.
However, this seems to only be a *cached* version, so I am unsure
exactly how reliable that link can be expected to remain over time
and have decided against using that one.