fix : 크레딧 추가 안됨

Author: ganggun

src/main/java/com/learnmore/legacy/domain/user/model/User.java

@@ -110,7 +110,7 @@ public void updateExp(Integer exp) {
     }
 
     public void updateCredit(Integer credit) {
-        this.credit = credit;
+        this.credit += credit;
     }
 
 }