[Perl/perl5] cb1faa: Use IsForeign throughout example

[brian d foy via perl5-changes]

  Branch: refs/heads/blead
  Home:   https://github.com/Perl/perl5
  Commit: cb1faabf2bbb9a718f9c0a3b7de82d8c25ce26ff
      
https://github.com/Perl/perl5/commit/cb1faabf2bbb9a718f9c0a3b7de82d8c25ce26ff
  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