Fix `mix cmd` examples in its moduledoc (#9969)
Fix `mix cmd` examples in its moduledoc (#9969)

- `mix cmd echo pwd` would only print literally `"pwd"` for each child app
- `mix cmd pwd` would print each child app's path
michalmuskala/elixir,kelvinst/elixir,ggcampinho/elixir,kimshrier/elixir,pedrosnk/elixir,pedrosnk/elixir,kelvinst/elixir,joshprice/elixir,ggcampinho/elixir,kimshrier/elixir,elixir-lang/elixir