fix spelling typo (#120062)

* fix spelling typo

* _index not nullable

This address the comment here: https://github.com/flutter/flutter/pull/120062#issuecomment-1421424676

packages/flutter/lib/src/widgets/editable_text.dart

@@ -5385,8 +5385,8 @@ class _UndoStack<T> {
 
   final List<T> _list = <T>[];
 
-  // The index of the current value, or null if the list is emtpy.
-  late int _index;
+  // The index of the current value, or -1 if the list is empty.
+  int _index = -1;
 
   /// Returns the current value of the stack.
   T? get currentValue => _list.isEmpty ? null : _list[_index];