Fixing the docstring for core/encrypt.

It's incorrectly advising 16834 as a good CPU cost parameter, but using
that value throws:

IllegalArgumentException N must be a power of 2 greater than 1
com.lambdaworks.crypto.SCrypt.scryptN (SCrypt.java:-2)

The value should be 16384. The 3 & 8 had been transposed.