CH04::privkeys: drop note about human-generated randomness

This was probaby the case on some JS-based private key websites, or when
using something like GPG, but it's unlikely to be the case on any modern
production software.

ch04.asciidoc

@@ -138,11 +138,8 @@ The first and most important step in generating keys is to find a secure
 source of entropy, or randomness. Creating a bitcoin key is essentially
 the same as "Pick a number between 1 and 2^256^." The exact method you
 use to pick that number does not matter as long as it is not predictable
-or repeatable. Bitcoin software uses the underlying operating system's
-random number generators to produce 256 bits of entropy (randomness).
-Usually, the OS random number generator is initialized by a human source
-of randomness, which is why you may be asked to wiggle your mouse around
-for a few seconds.
+or repeatable. Bitcoin software uses cryptographically-secure random
+number generators to produce 256 bits of entropy (randomness).
 
 More precisely, the private key can be any number between +0+ and +n -
 1+ inclusive, where n is a constant (n = 1.1578 * 10^77^, slightly less