add cpp in code snippet

Set cpp in the fence area to improve reference website support.
`arduino` (or `ino`) are also valid styles for our reference website and will be styled accordingly.

Author: Francesco Stasi

docs/readme.md

@@ -41,7 +41,7 @@ MKRIoTCarrier yourName; //In the examples we name it as *carrier*
 
 Intialization example sketch
 
-```
+```cpp
 #include <Arduino_MKRIoTCarrier.h>
 MKRIoTCarrier carrier;