Update obsolete IO::Path names to IO::Spec

IO::Path::Unix -> IO::Spec::Unix and so on.
It fixes #931, i.e. 404 links.
Still as Coke said #931 (comment) the whole section can be revisited.
grizzlysmit++

doc/Type/IO/Path.pod6

@@ -19,8 +19,8 @@ The current working directory is available as an IO::Path object in C<$*CWD>.
 
 The behavior of C<IO::Path> is dependent on the operating system it runs on;
 to get reproducible behavior across operating systems, you can use one of its
-subclasses instead: L<IO::Path::Unix>, L<IO::Path::Win32>,
-L<IO::Path::Cygwin>, L<IO::Path::QNX>.
+subclasses instead: L<IO::Spec::Unix>, L<IO::Spec::Win32>,
+L<IO::Spec::Cygwin>, L<IO::Spec::QNX>.
 
 The rest of this document silently assumes Unix semantics in its examples,
 unless when stated otherwise.