Show how to get exit code from Proc::Async

Possibly resolves issue #1465.

Author: AlexDaniel

doc/Type/Proc/Async.pod6

@@ -28,7 +28,7 @@ standard output and error handles, and optionally write to its standard input.
             say ‘stderr: ’, $_
         }
         whenever $proc.start {
-            say ‘Proc finished’;
+            say ‘Proc finished. Exit code: ’, .exitcode;
             done # gracefully jump from the react block
         }
         whenever $proc.print: “I\n♥\nCamelia\n” {
@@ -66,7 +66,7 @@ line: and
 line: Camelia
 line: ♥
 line: I
-Proc finished
+Proc finished. Exit code: 0
 Program finished
 =end code