Update ExtUI path in config comment (#19908)

Marlin/Configuration.h

@@ -2196,7 +2196,7 @@
 
 //
 // Third-party or vendor-customized controller interfaces.
-// Sources should be installed in 'src/lcd/extensible_ui'.
+// Sources should be installed in 'src/lcd/extui'.
 //
 //#define EXTENSIBLE_UI