fix arity of crypto:hash_update/2 in deprecation warnings

When compiling code using deprecated crypto functions that should be
replaced with crypto:hash_update/2, the deprecation warnings were saying
"use crypto:hash_update/3", with the wrong arity of 3, instead of "use
crypto:hash_update/2".

The problem was reported in this email message to erlang-bugs:

http://erlang.org/pipermail/erlang-bugs/2014-March/004281.html