Add 's' and 'S' to list of ANSI C portable filename characters.

For: RT #127783.  Thanks to Jim Davis for report.

Author: jkeenan

pod/perlport.pod

@@ -376,8 +376,8 @@ semantics for that.  Let the operating system sort it out.
 
 The I<portable filename characters> as defined by ANSI C are
 
- a b c d e f g h i j k l m n o p q r t u v w x y z
- A B C D E F G H I J K L M N O P Q R T U V W X Y Z
+ a b c d e f g h i j k l m n o p q r s t u v w x y z
+ A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
  0 1 2 3 4 5 6 7 8 9
  . _ -