Update static/app.js

static/app.js

@@ -11,7 +11,7 @@ var board = Chessboard("board", {
 });
 
 function updateInfo() {
-  document.getElementById("fen").textContent = "FEN: " + board.fen();
+  document.getElementById('fen').value = board.fen();
   let pos = board.position();
   let pieces = Object.keys(pos).length;
 }