refactor: 优化用户可见错误提示

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

@@ -9,6 +9,6 @@ import lombok.Data;
 @Data
 public class UpdateStandardMindMapRequest {
 
-    @NotBlank(message = "思维导图大纲不可为空")
+    @NotBlank(message = "请填写思维导图大纲")
     private String outline;
 }