Fix typo'd missing : in .nominal_type signature (#4788)
Fix typo'd missing : in .nominal_type signature (#4788)

The Parameter.nominal_type method had a signature that clearly
intended to constrain the invocant to be :D, but was missing the
trailing :

Fixing this issue causes a TODO'ed Roast test to pass.
labster/rakudo,tbrowder/rakudo,rakudo/rakudo,salortiz/rakudo,salortiz/rakudo,MasterDuke17/rakudo,rakudo/rakudo,salortiz/rakudo,rakudo/rakudo,rakudo/rakudo,tbrowder/rakudo,salortiz/rakudo,MasterDuke17/rakudo,labster/rakudo,MasterDuke17/rakudo,tbrowder/rakudo,rakudo/rakudo,MasterDuke17/rakudo,labster/rakudo,MasterDuke17/rakudo,salortiz/rakudo,labster/rakudo,salortiz/rakudo,labster/rakudo,labster/rakudo,tbrowder/rakudo,tbrowder/rakudo,MasterDuke17/rakudo,rakudo/rakudo,tbrowder/rakudo