[Error] 기본적이지만 까먹을 수 있는 에러

## 어떤 에러가 발생했나요?
- node server가 실행이 되지 않음

### 에러가 발생한 폴더
- /server/index.js

### 에러 메시지
```
/Users/jortier/code_states_project/project/triplus/server/node_modules/express/lib/router/index.js:458
throw new TypeError('Router.use() requires a middleware function but got a ' + gettype(fn))
^

TypeError: Router.use() requires a middleware function but got a Object
at Function.use (/Users/jortier/code_states_project/project/triplus/server/node_modules/express/lib/router/index.js:458:13)
at Function. (/Users/jortier/code_states_project/project/triplus/server/node_modules/express/lib/application.js:220:21)
at Array.forEach ()
at Function.use (/Users/jortier/code_states_project/project/triplus/server/node_modules/express/lib/application.js:217:7)
at Object. (/Users/jortier/code_states_project/project/triplus/server/index.js:48:5)
at Module._compile (node:internal/modules/cjs/loader:1101:14)
at Object.Module._extensions..js (node:internal/modules/cjs/loader:1153:10)
at Module.load (node:internal/modules/cjs/loader:981:32)
at Function.Module._load (node:internal/modules/cjs/loader:822:12)
at Function.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:81:12)
```

### 에러 원인
router middleware에서 function을 가져오지 못하고 object를 가져옴

### 에러 해결 방법
```javascript
module.exports = router;
```
export를 안해줘서 발생하는 에러였다.
까먹지말고 꼭 해주자.