Fix avatar url type of update avatar (#575)

Author: huyuwei1996

backend/app/admin/crud/crud_user.py

@@ -142,7 +142,7 @@ async def update_avatar(self, db: AsyncSession, input_user: int, avatar: AvatarP
         :param avatar: 头像地址
         :return:
         """
-        return await self.update_model(db, input_user, {'avatar': avatar.url})
+        return await self.update_model(db, input_user, {'avatar': str(avatar.url)})
 
     async def delete(self, db: AsyncSession, user_id: int) -> int:
         """