Show how to get a PID from .ready

This example is already overcomplicated, so adding more stuff to it is
a bad idea. Still, demonstrating how to get a pid is useful, so we
have to show it (but maybe this example has to be split in two).

Author: AlexDaniel

doc/Type/Proc/Async.pod6

@@ -28,6 +28,9 @@ react {
     whenever $proc.stderr { # chunks
         say ‘stderr: ’, $_
     }
+    whenever $proc.ready {
+        say ‘PID: ’, $_ # Only in Rakudo 2018.04 and newer, otherwise Nil
+    }
     whenever $proc.start {
         say ‘Proc finished. Exit code: ’, .exitcode;
         done # gracefully jump from the react block