update playground link

Author: Rotzbua

README.md

@@ -29,7 +29,7 @@ https://groups.google.com/a/arduino.cc/forum/#!forum/developers
 Installation
 ------------
 Detailed instructions for installation in popular operating systems.  
-For Linux: https://www.arduino.cc/en/Guide/Linux (see also the Arduino playground page http://www.arduino.cc/playground/Learning/Linux)   
+For Linux: https://www.arduino.cc/en/Guide/Linux (see also the Arduino playground page https://playground.arduino.cc/Learning/Linux)   
 For macOS X: https://www.arduino.cc/en/Guide/MacOSX   
 For Windows: https://www.arduino.cc/en/Guide/Windows