fix: typo in page-variable.md (#40)

docs/developer-guide/theme/page-variable.md

@@ -397,14 +397,14 @@ ${nextPost.attribute}
 
 ```html
 <#if nextPost??>
-	<a href="${nextPost.fullPath!}">上一篇：${nextPost.title!}</a>
+	<a href="${nextPost.fullPath!}">下一篇：${nextPost.title!}</a>
 </#if>
 ```
 
 输出：
 
 ```html
-<a href="http://localhost:8090/archives/url3">上一篇：title3</a>
+<a href="http://localhost:8090/archives/url3">下一篇：title3</a>
 ```
 
 ### categories（List）
@@ -1351,4 +1351,4 @@ ${keyword!}
   	2021年3月7日：内容2
   </li>
 </ul>
-```
+```