[perl6/specs] b4b49f: Improve wording about substr-rw aliasing

[GitHub]

  Branch: refs/heads/master
  Home:   https://github.com/perl6/specs
  Commit: b4b49fcd52a9731213ad135788df89d67b5e235c
      
https://github.com/perl6/specs/commit/b4b49fcd52a9731213ad135788df89d67b5e235c
  Author: Paul Cochrane <p...@liekut.de>
  Date:   2015-06-09 (Tue, 09 Jun 2015)

  Changed paths:
    M S32-setting-library/Str.pod

  Log Message:
  -----------
  Improve wording about substr-rw aliasing

Thanks to itz++ for pointing out the sentence's meaning.