[book] Fix `p_poly` to match implementation; specify synthetic blinding factor `f` construction

p_poly is constructed as a linear combination of q_prime and the q_polys in steps 18 and 19 of the protocol description. Previously, the expression used in the protocol description did not match the implementation:

https://github.com/zcash/halo2/blob/5678a506cbec60d593a21ff618d09bed48abf7f2/halo2_proofs/src/poly/multiopen/prover.rs#L105-L113
https://github.com/zcash/halo2/blob/5678a506cbec60d593a21ff618d09bed48abf7f2/halo2_proofs/src/poly/multiopen/verifier.rs#L120-L129

zcash / halo2

[book] Fix `p_poly` to match implementation; specify synthetic blinding factor `f` construction #777