feat(example): Use CC0 license for examples

examples/storage/spiffs_test/main/test_main.c

@@ -1,3 +1,12 @@
+/* SPIFFS test example
+
+   This example code is in the Public Domain (or CC0 licensed, at your option.)
+
+   Unless required by applicable law or agreed to in writing, this
+   software is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+   CONDITIONS OF ANY KIND, either express or implied.
+*/
+
 #include "esp_spiffs.h"
 #include "esp_ssc.h"
 #include "esp_system.h"