HR, App. D.5, minor correction

[Ralf Hinze]

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

=09showsPrec d (Leaf m) =3D showParen (d >=3D 10) showStr

should be replaced by

=09showsPrec d (Leaf m) =3D showParen (d > app_prec) showStr

Cheers, Ralf