updateEditingValueWithDeltas snippet docs fix (#99570)

Just a docs change for a broken tool snippet.

packages/flutter/lib/src/services/text_input.dart

@@ -1195,16 +1195,23 @@ abstract class DeltaTextInputClient extends TextInputClient {
   /// to the client's editing state. A change is any mutation to the raw text
   /// value, or any updates to the selection and/or composing region.
   ///
-  /// Here is an example of what implementation of this method could look like:
   /// {@tool snippet}
+  /// This example shows what an implementation of this method could look like.
+  ///
+  /// ```dart
+  /// TextEditingValue? _localValue;
   /// @override
   /// void updateEditingValueWithDeltas(List<TextEditingDelta> textEditingDeltas) {
-  ///   TextEditingValue newValue = _previousValue;
+  ///   if (_localValue == null) {
+  ///     return;
+  ///   }
+  ///   TextEditingValue newValue = _localValue!;
   ///   for (final TextEditingDelta delta in textEditingDeltas) {
   ///     newValue = delta.apply(newValue);
   ///   }
   ///   _localValue = newValue;
   /// }
+  /// ```
   /// {@end-tool}
   void updateEditingValueWithDeltas(List<TextEditingDelta> textEditingDeltas);
 }