course fetchOne

GET /api/courses/:id

返回

{
  "code": 1,
  "msg": "SUCCESS",
  "data": {
    "id": "5b29e61cca51891746b20206",
    "title": "title",
    "coverImg": "asdasdsad",
    "direction": 1,
    "type": [0, 1, 2],
    "difficulty": 0,
    "price": 10,
    "sections": [
      {
        "title": "title"
      }
    ]
  }
}