Fix PerlIO::scalar fileno() documentation as suggested in:

Subject: [perl #40245] POD error for PerlIO::scalar in 5.8.8 
From: "Shawn Boyette" (via RT) <perlbug-followup@perl.org>
Message-ID: <rt-3.5.HEAD-31263-1156749685-634.40245-75-0@perl.org>

p4raw-id: //depot/perl@28766

ext/PerlIO/scalar/scalar.pm

@@ -30,7 +30,7 @@ or
 A filehandle is opened but the file operations are performed "in-memory"
 on a scalar variable.  All the normal file operations can be performed
 on the handle. The scalar is considered a stream of bytes.  Currently
-fileno($fh) returns C<undef>.
+fileno($fh) returns -1.
 
 =head1 IMPLEMENTATION NOTE