doc: update controller length extension method deprecation message

[realth000]

Description

V11 deprecates the length extension method on controller and the deprecation message suggests to use selection.baseOffset instead.

However it will cause assertion failure for the following code

  void insertEmbeddable(Embeddable embed) {
    final position = selection.baseOffset;
    this
      ..replaceText(
        position, // In former versions, here is `index`.
        position, // In former versions, here is `length`.
        embed,
        null,
      )
      ..moveCursorToPosition(position + 1);
  }

Assertion failure:

flutter: ┌──────────────────────────────────────────────────────────────────────────────────────────────────────────────
flutter: │ [error] | 3:28:44 815ms | 
flutter: │ 'package:flutter_quill/src/document/nodes/container.dart': Failed assertion: line 138 pos 12: 'index == 0 || (index > 0 && index < length)': is not true.
flutter: │ StackTrace: #0      _AssertionError._doThrowNew (dart:core-patch/errors_patch.dart:63:4)
flutter: │ #1      _AssertionError._throwNew (dart:core-patch/errors_patch.dart:45:5)
flutter: │ #2      QuillContainer.insert (package:flutter_quill/src/document/nodes/container.dart:138:12)
flutter: │ #3      Document.compose (package:flutter_quill/src/document/document.dart:451:15)
flutter: │ #4      Document.insert (package:flutter_quill/src/document/document.dart:88:5)
flutter: │ #5      Document.replace (package:flutter_quill/src/document/document.dart:144:17)
flutter: │ #6      QuillController.replaceText (package:flutter_quill/src/controller/quill_controller.dart:284:24)
flutter: │ #7      BBCodeExt.insertEmbeddable (package:flutter_bbcode_editor/src/editor_controller.dart:120:9)
flutter: │ #8      BBCodeEditorToolbarImageButton.build.<anonymous closure> (package:flutter_bbcode_editor/src/tags/image/image_button.dart:205:20)
flutter: │ <asynchronous suspension>
flutter: │ }

I think it should be a typo as the getter actually now is selection.extentOffset - selection.baseOffset.

This PR updates the deprecation message on length method to match its implementation.

Related Issues

Type of Change

• ✨ Feature: New functionality without breaking existing features.
• 🛠️ Bug fix: Resolves an issue without altering current behavior.
• 🧹 Refactor: Code reorganization, no behavior change.
• ❌ Breaking: Alters existing functionality and requires updates.
• 🧪 Tests: New or modified tests
• 📝 Documentation: Updates or additions to documentation.
• 🗑️ Chore: Routine tasks, or maintenance.
• ✅ Build configuration change: Build/configuration changes.

[EchoEllet]

Thank you!