Remove comment reference

The comment references an interface element that doesn't exist anymore.

Author: jacobhylen

libraries/Arduino_LED_Matrix/examples/LivePreview/LivePreview.ino

@@ -1,7 +1,6 @@
 /*
   This sketch allows live editing of the matrix pixels using WebSerial
   To test, head to https://ledmatrix-editor.arduino.cc
-  press 'p' and select 'UNO R4 WiFi' board from the list.
 
   Don't forget to close any serial monitor already opened.
 */