Clean code

- Fix typo to make them more understandable when reading the code
 
Example :
> Rename `s` to `servce`

```go
func (s *Service) GetUserByID(ctx context.Context, id string) (*User, error) {
    return s.Repo.GetByID(ctx, id)
}
```

- Clean `main.go`
  - Remove comment, log print, etc.