Fix parameter type of `make clean` in mix.exs

[tywhisky]

Hello, I have found a little bug when i compile elixir-captcha on Windows. And i fixed it.

With the wrong argument 'clean' ("clean" correctly), Elixir will raise:

** (ArgumentError) all arguments for System.cmd/3 must be binaries (elixir 1.13.4) lib/system.ex:1035: System.cmd/3