Update RMTLoopback.ino (#3823)

BUGFIX: avoids assertion in xEventGroupWaitBits()
(/home/runner/work/esp32-arduino-lib-builder/esp32-arduino-lib-builder/esp-idf/components/freertos/event_groups.c:350 (xEventGroupWaitBits)- assert failed!)

libraries/ESP32/examples/RMT/RMTLoopback/RMTLoopback.ino

@@ -16,6 +16,7 @@ static EventGroupHandle_t events;
 void setup() 
 {
     Serial.begin(115200);
+    events = xEventGroupCreate();
     
     if ((rmt_send = rmtInit(18, true, RMT_MEM_64)) == NULL)
     {