Update mix.md (#938)
Update mix.md (#938)

Changing to the right file extension
nscyclone/elixir-school,doomspork/elixir-school,doomspork/elixir_school,doomspork/elixir_school,nscyclone/elixir-school,doomspork/elixir-school,doomspork/elixir_school,erickgnavar/elixir-school,erickgnavar/elixir-school,doomspork/elixir-school,erickgnavar/elixir-school,nscyclone/elixir-school,psantos10/elixir-school,psantos10/elixir-school,psantos10/elixir-school,nscyclone/elixir-school