Make shutdown docs compilable (#330)

README.md

@@ -436,7 +436,8 @@ func main() {
     <-c
 
     // Create a deadline to wait for.
-    ctx, cancel := context.WithTimeout(ctx, wait)
+    ctx, cancel := context.WithTimeout(context.Background(), wait)
+    defer cancel()
     // Doesn't block if no connections, but will otherwise wait
     // until the timeout deadline.
     srv.Shutdown(ctx)