Change hold to shift from C

public/js/app.js

@@ -143,8 +143,7 @@ function setupKeyboardControls() {
           network.sendHardDrop();
           e.preventDefault();
           break;
-        case 'c':
-        case 'C':
+        case 'Shift':
           network.sendHold();
           e.preventDefault();
           break;