[S26] fixed blatant p5ism

Commit prepared by Damian Conway.

Author: Carl Masak

S26-documentation.pod

@@ -1548,8 +1548,8 @@ nestable block comments in Perl 6:
 
 =begin code
     =begin comment
-    for my $file (@files) {
-        system("rm -rf $file");
+    for @files -> $file {
+        shell("rm -rf $file");
     }
     =end comment
 =end code