Add missing =for code on one paragraph (#1361)

On https://docs.perl6.org/language/io-guide this bit was rendering as plain body
text. I decided against putting `=begin code`..`=end code` around the whole area
in case someone else might want to add exposition in between later.

Author: flussence

doc/Language/io-guide.pod6

@@ -244,6 +244,7 @@ my $fh = open $*SPEC.catpath: '', 'foo/bar', $file;
 my $data = $fh.slurp;
 $fh.close;
 
+=for code :skip-test
 # RIGHT! Use IO::Path to do all the dirty work
 my $data = 'foo/bar'.IO.add($file).slurp;