docs(issue_template): 在问题模板中添加YAML前后分隔符

为书籍问题模板文件添加标准的YAML前后分隔符(---)，以符合GitHub issue模板的规范格式

.github/ISSUE_TEMPLATE/book_issue.md

@@ -1,3 +1,4 @@
+---
 name: Happy-LLM 问题反馈 / Textbook Issue Report
 description: 报告 Happy-LLM 中的代码/内容问题 / Report code/content issues in textbook
 title: "[问题/Issue] 章节X.X：简短问题描述 / ChapterX.X: Brief description"
@@ -63,4 +64,5 @@ body:
       description: 请确认以下事项 / Please confirm the following
       options:
         - label: 此问题未在过往Issue中被报告过 / This issue hasn't been reported before
-          required: true
+          required: true
+---