CH04::privkeys: update for HD wallets

- Previously said privkeys were numbers picked at random.  Updated to
  say "derived from numbers picked at random".

ch04.asciidoc

@@ -75,7 +75,8 @@ from the private key, so storing only the private key is also possible.
 pairs")))((("elliptic curve cryptography")))((("cryptography", "elliptic
 curve cryptography")))A bitcoin wallet contains a collection of key
 pairs, each consisting of a private key and a public key. The private
-key (k) is a number, usually picked at random. From the private key, we
+key (k) is a number, usually derived from a number picked at random.
+From the private key, we
 use elliptic curve multiplication, a one-way cryptographic function, to
 generate a public key (K). From the public key (K), we use a one-way
 cryptographic hash function to generate a Bitcoin address (A). In this