add where clause to MAIN example

Author: gfldex

doc/Language/functions.pod

@@ -729,7 +729,7 @@ and it's signature is the means by which command line arguments can be parsed.
 Multi methods are supported and a usage method is automatically generated and
 displayed if no command line arguments are provided.
 
-    sub MAIN ( Int :$length = 24, :file($data) = 'file.dat', Bool :$verbose ) {
+    sub MAIN ( Int :$length = 24, :file($data) where { .IO.f // die "file not found in $*CWD" } = 'file.dat', Bool :$verbose ) {
         say $length if $length.defined;
         say $data   if $data.defined;
         say 'Verbosity ', ($verbose ?? 'on' !! 'off');