kqueue: add close() calls to man page example

There is no real need to close descriptors before a process exits, but
these close calls demonstrate by example that kqueue descriptors occupy
the same namespace as other file descriptors.

Reviewed by:	fernape, markj
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39376

lib/libc/sys/kqueue.2

@@ -773,6 +773,10 @@ main(int argc, char **argv)
 		printf("Something was written in '%s'\en", argv[1]);
 	}
     }
+
+    /* kqueues are destroyed upon close() */
+    (void)close(kq);
+    (void)close(fd);
 }
 .Ed
 .Sh ERRORS