FIxed the timer and the example

examples/yes/main.orc

@@ -0,0 +1,9 @@
+import system::async::(set_timer, yield)
+import system::io::(readln, println)
+import std::exit_status
+
+const main := (
+  set_timer true 1 (println "y" yield) \cancel.
+    readln \a.
+      cancel exit_status::success
+)