Update README.md

Fix `php fatal error call to unknown function addCustomMapping` when using example code from section "Custom Control Mapping", by changing `addCustomMapping` to `addCustomControlMapping`.

Author: authanram

README.md

@@ -906,7 +906,7 @@ $menu = (new CliMenuBuilder)
     ->addItem('Item 1')
     ->build();
 
-$menu->addCustomMapping("x", $exit);
+$menu->addCustomControlMapping("x", $exit);
 
 $menu->open();
 ```
@@ -928,7 +928,7 @@ $menu = (new CliMenuBuilder)
     ->build();
 
 // Now, pressing Uppercase C (it's case sensitive) will call $myCallback
-$menu->addCustomMapping('C', $myCallback);
+$menu->addCustomControlMapping('C', $myCallback);
 
 $menu->open();
 ```