[perl6/specs] 007993: [S14] use :U instead of "where {!.defined}"

[GitHub]

  Branch: refs/heads/master
  Home:   https://github.com/perl6/specs
  Commit: 00799304600458dd6a37995f2884146cf28593ec
      
https://github.com/perl6/specs/commit/00799304600458dd6a37995f2884146cf28593ec
  Author: Moritz Lenz <mor...@faui2k3.org>
  Date:   2012-05-08 (Tue, 08 May 2012)

  Changed paths:
    M S14-roles-and-parametric-types.pod

  Log Message:
  -----------
  [S14] use :U instead of "where {!.defined}"