Correct opening brace in implicits in pprinter

Before:
    Prelude.Basics.id : (a : Type} -> (__pi_arg : a) -> a

After:
    Prelude.Basics.id : {a : Type} -> (__pi_arg : a) -> a
Merge pull request #1097 from david-christiansen/pprint

Correct opening brace in implicits in pprinter