pidfd_open.2: Close the pidfd in EXAMPLE

Close the PID file descriptor in the example program, to hint to
the reader that like every other kind of file descriptor, a PID FD
should be closed.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/pidfd_open.2

@@ -256,6 +256,7 @@ main(int argc, char *argv[])
     printf("Events (0x%x): POLLIN is %sset\en", pollfd.revents,
             (pollfd.revents & POLLIN) ? "" : "not ");
 
+    close(pidfd);
     exit(EXIT_SUCCESS);
 }
 .fi