[ch6] Use n instead of p for group order of EC

This is to match existing common conventions.
By convention, n, not p, is used to indicate the prime group order of the EC.
This is currently extra confusing because p *is* (by convention) used to indicate the order of the EC's finite field.

References:
https://github.com/bitcoin-core/secp256k1/blob/master/src/ecdsa_impl.h
http://www.secg.org/sec2-v2.pdf
https://en.wikipedia.org/wiki/Elliptic_Curve_Digital_Signature_Algorithm

ch06.asciidoc

@@ -650,7 +650,7 @@ The temporary key pair is based on a random number _k_, which is used as the tem
 
 From there, the algorithm calculates the _S_ value of the signature, such that:
 
-_S_ = __k__^-1^ (__Hash__(__m__) + __dA__ * __R__) _mod p_
+_S_ = __k__^-1^ (__Hash__(__m__) + __dA__ * __R__) _mod n_
 
 where:
 
@@ -658,7 +658,7 @@ where:
 * _R_ is the x coordinate of the ephemeral public key
 * _dA_ is the signing private key
 * _m_ is the transaction data
-* _p_ is the prime order of the elliptic curve
+* _n_ is the prime order of the elliptic curve
 
 Verification is the inverse of the signature generation function, using the _R_, _S_ values and the public key to calculate a value _P_, which is a point on the elliptic curve (the ephemeral public key used in signature creation):