[Perl/perl5] 27d900: Use IsForeign throughout example

[brian d foy via perl5-changes]

  Branch: refs/heads/briandfoy-isforeign
  Home:   https://github.com/Perl/perl5
  Commit: 27d9000cdb43efbb58d2f2027f51def1b3aa7596
      
https://github.com/Perl/perl5/commit/27d9000cdb43efbb58d2f2027f51def1b3aa7596
  Author: brian d foy <brian.d....@gmail.com>
  Date:   2021-03-05 (Fri, 05 Mar 2021)

  Changed paths:
    M pod/perlunicode.pod

  Log Message:
  -----------
  Use IsForeign throughout example

The example used both IsForeign and Is_Foreign, so I chose the one without the 
underscore