adding SPDX to arduino code

and none since it is for Pico only and not an available platform

PiCowbell_Adalogger_Examples/Arduino_PiCowbell_Adalogger_Example/Arduino_PiCowbell_Adalogger_Example.ino

@@ -1,3 +1,7 @@
+// SPDX-FileCopyrightText: 2023 Liz Clark for Adafruit Industries
+//
+// SPDX-License-Identifier: MIT
+
 const int _MISO = 16;
 const int _MOSI = 19;
 const int _CS = 17;