improve MQTT example, remove topic leading slash

The mqtt topic forward slash '/' counts towards tree levels in spec, thus best practise is to not lead topic paths with a slash.

examples/protocols/mqtt/tcp/main/app_main.c

@@ -23,6 +23,9 @@
 
 static const char *TAG = "mqtt_example";
 
+// MQTT Test Topics
+static const char * topic_qos0 = "topic/qos0";
+static const char * topic_qos1 = "topic/qos1";
 
 static void log_error_if_nonzero(const char *message, int error_code)
 {
@@ -50,16 +53,16 @@ static void mqtt_event_handler(void *handler_args, esp_event_base_t base, int32_
     switch ((esp_mqtt_event_id_t)event_id) {
     case MQTT_EVENT_CONNECTED:
         ESP_LOGI(TAG, "MQTT_EVENT_CONNECTED");
-        msg_id = esp_mqtt_client_publish(client, "/topic/qos1", "data_3", 0, 1, 0);
+        msg_id = esp_mqtt_client_publish(client, topic_qos1, "data_3", 0, 1, 0);
         ESP_LOGI(TAG, "sent publish successful, msg_id=%d", msg_id);
 
-        msg_id = esp_mqtt_client_subscribe(client, "/topic/qos0", 0);
+        msg_id = esp_mqtt_client_subscribe(client, topic_qos0, 0);
         ESP_LOGI(TAG, "sent subscribe successful, msg_id=%d", msg_id);
 
-        msg_id = esp_mqtt_client_subscribe(client, "/topic/qos1", 1);
+        msg_id = esp_mqtt_client_subscribe(client, topic_qos1, 1);
         ESP_LOGI(TAG, "sent subscribe successful, msg_id=%d", msg_id);
 
-        msg_id = esp_mqtt_client_unsubscribe(client, "/topic/qos1");
+        msg_id = esp_mqtt_client_unsubscribe(client, topic_qos1);
         ESP_LOGI(TAG, "sent unsubscribe successful, msg_id=%d", msg_id);
         break;
     case MQTT_EVENT_DISCONNECTED:
@@ -68,7 +71,7 @@ static void mqtt_event_handler(void *handler_args, esp_event_base_t base, int32_
 
     case MQTT_EVENT_SUBSCRIBED:
         ESP_LOGI(TAG, "MQTT_EVENT_SUBSCRIBED, msg_id=%d", event->msg_id);
-        msg_id = esp_mqtt_client_publish(client, "/topic/qos0", "data", 0, 0, 0);
+        msg_id = esp_mqtt_client_publish(client, topic_qos0, "data", 0, 0, 0);
         ESP_LOGI(TAG, "sent publish successful, msg_id=%d", msg_id);
         break;
     case MQTT_EVENT_UNSUBSCRIBED: