bump to 3 seconds for stability

Author: vladak

examples/esp32spi/minimqtt_pub_sub_nonblocking_esp32spi.py

@@ -109,4 +109,4 @@ def message(client, topic, message):
     print("Sending photocell value: %d" % photocell_val)
     mqtt_client.publish(default_topic, photocell_val)
     photocell_val += 1
-    time.sleep(0.5)
+    time.sleep(3)