CH05::HD derivation: extended keys

Previous text said that extended keys were 512 bit concatenations, but
BIP32 includes extra data, including the depth, fingerprint, child
number, and a null byte for private keys.  Update to be less precise but
more accurate.

ch05.asciidoc

@@ -1058,7 +1058,7 @@ _extended key_. The term "extended key" could also be thought of as
 "extensible key" because such a key can be used to derive children.
 
 Extended keys are stored and represented simply as the concatenation of
-the 256-bit key and 256-bit chain code into a 512-bit sequence. There
+the key and chain code. There
 are two types of extended keys. An extended private key is the
 combination of a private key and chain code and can be used to derive
 child private keys (and from them, child public keys). An extended