HR, App. D.5, minor correction

Hi Simon, here is a minor correction to the May version of the HR. In appendix D.5 there is a table with an example for derived instances: for reasons of consistency the line showsPrec d (Leaf m) = showParen (d >= 10) showStr should be replaced by showsPrec d (Leaf m) = showParen (d > app_prec) showStr Cheers, Ralf