修复拆分TXT时最后一章缺失的问题

[Richasy]

Close #58

修复拆分TXT时最后一章缺失的问题

PR type

What is the purpose of this PR?

• Bug fix

What is the current behavior?

拆分txt小说时，因为使用了错误的标题变量，导致最后一章无法被添加到标题列表中

What is the new behavior?

修复这个问题

PR checklist

Please check that your PR meets the following requirements:

• [x] App successfully launched
• [x] File headers have been added to all source files
• [x] NOT Contains breaking updates

Remark

替换了默认的分章表达式