Add comment at top of 1991/fine/demo.sh

It seems like if I'm going to add a demo script it might be a good idea
to have a comment briefly explaining what the script does but in such a
way that does not spoil the entry.

1991/fine/demo.sh

@@ -1,4 +1,7 @@
 #!/usr/bin/env bash
+#
+# demo.sh - script to demonstrate IOCCC entry 1991/fine
+#
 
 make all || exit 1