refactor: 优化用户可见错误提示

src/main/java/com/guo/learningprogresstracker/dto/request/CreateFragmentsRequest.java

@@ -16,6 +16,6 @@ public class CreateFragmentsRequest {
     /**
      * 残片内容,学习内容的描述
      */
-    @NotBlank(message = "啊？无字天书？")
+    @NotBlank(message = "请填写学习内容")
     private String content;
 }