Remove unexplained acronym.
Remove unexplained acronym.

I don't think we can assume that everyone knows that PSK is, and
with the extensive acronym soup in use in Varnish it is hard
for the user to know if we have overloaded it.

feld/Varnish-Cache,gquintard/Varnish-Cache,gquintard/Varnish-Cache,feld/Varnish-Cache,feld/Varnish-Cache,gquintard/Varnish-Cache,gquintard/Varnish-Cache,feld/Varnish-Cache,feld/Varnish-Cache