fix: underscore markdown syntax assertion

[LucasXu0]

[ERROR:flutter/runtime/dart_vm_initializer.cc(41)] Unhandled Exception: RangeError (end): Invalid value: Not in inclusive range 4..5: -1
#0      RangeError.checkValidRange (dart:core/errors.dart:365:9)
#1      _StringBase.substring (dart:core-patch/string_patch.dart:418:27)
#2      _OpIterator.next (package:appflowy_editor/src/core/document/text_delta.dart:591:23)
#3      Delta.compose (package:appflowy_editor/src/core/document/text_delta.dart:277:33)
#4      TextTransaction.compose.<anonymous closure> (package:appflowy_editor/src/core/transform/transaction.dart:568:59)
#5      ListBase.fold (dart:collection/list.dart:202:22)
#6      TextTransaction.compose (package:appflowy_editor/src/core/transform/transaction.dart:568:22)
#7      Transaction.operations (package:appflowy_editor/src/core/transform/transaction.dart:22:7)
#8      EditorState._applyTransactionInLocal (package:appflowy_editor/src/editor_state.dart:549:34)
#9      EditorState.apply (package:appflowy_editor/src/editor_state.dart:307:7)
#10     handleFormatByWrappingWithSingleCharacter (package:appflowy_editor/src/editor/editor_component/service/shortcuts/character/format_single_character/format_single_character.dart:107:15)
#11     formatUnderscoreToItalic.<anonymous closure> (package:appflowy_editor/src/editor/editor_component/service/shortcuts/character/format_single_character/format_italic.dart:17:35)
#12     executeCharacterShortcutEvent (package:appflowy_editor/src/editor/editor_component/service/ime/character_shortcut_event_helper.dart:14:36)
<asynchronous suspension>
#13     onInsert (package:appflowy_editor/src/editor/editor_component/service/ime/delta_input_on_insert_impl.dart:16:21)
<asynchronous suspension>
#14     KeyboardServiceWidgetState.initState.<anonymous closure> (package:appflowy_editor/src/editor/editor_component/service/keyboard_service_widget.dart:65:38)
<asynchronous suspension>
#15     NonDeltaTextInputService.apply (package:appflowy_editor/src/editor/editor_component/service/ime/non_delta_input_service.dart:49:9)
<asynchronous suspension>

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.79%. Comparing base (98a49ad) to head (109ced9).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #826      +/-   ##
==========================================
- Coverage   74.85%   74.79%   -0.07%     
==========================================
  Files         301      301              
  Lines       14204    14199       -5     
==========================================
- Hits        10633    10620      -13     
- Misses       3571     3579       +8     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.