don't use .payload for a "handle all exceptions" example

because payload only exists on X::AdHoc. Use `.message` instead,
which every Exception has, and which just stringifies the payload on
an X::AdHoc anyway

doc/Language/exceptions.pod6

@@ -72,7 +72,7 @@ almost the same information as the normal backtrace printer.
 
     CATCH {
          default {
-             $*ERR.say: .payload;
+             $*ERR.say: .message;
              for .backtrace.reverse {
                  next if .file.starts-with('SETTING::');
                  next unless .subname;