CH04::privkeys: Hash digests aren't numbers

We just choose to interpet them that way

ch04.asciidoc

@@ -150,7 +150,8 @@ than 2^256^) defined as the order of the elliptic curve used in bitcoin
 256-bit number and check that it is less than +n+. In programming terms,
 this is usually achieved by feeding a larger string of random bits,
 collected from a cryptographically secure source of randomness, into the
-SHA256 hash algorithm, which will conveniently produce a 256-bit number.
+SHA256 hash algorithm, which will conveniently produce a 256-bit value
+that can be interpreted as a number.
 If the result is less than +n+, we have a suitable private key.
 Otherwise, we simply try again with another random number.