Add delay to manualAcknowledgements example (#953)

#949
- Add delay to relax how often available() is called to let the radio internally do its thing
* Update examples/ManualAcknowledgements/ManualAcknowledgements.ino

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

examples/ManualAcknowledgements/ManualAcknowledgements.ino

@@ -129,6 +129,7 @@ void loop() {
       while (!radio.available()) {             // wait for response
         if (millis() - start_timeout > 200)    // only wait 200 ms
           break;
+        delayMicroseconds(200);  // relax probing of available()
       }
       unsigned long end_timer = micros();  // end the timer
       radio.stopListening();               // put back in TX mode