Update plug.md (#1181)
Update plug.md (#1181)

Add missing `a`
erickgnavar/elixir-school,nscyclone/elixir-school,nscyclone/elixir-school,psantos10/elixir-school,nscyclone/elixir-school,erickgnavar/elixir-school,doomspork/elixir_school,doomspork/elixir_school,psantos10/elixir-school,doomspork/elixir-school,doomspork/elixir-school,doomspork/elixir-school,erickgnavar/elixir-school,psantos10/elixir-school,doomspork/elixir_school,nscyclone/elixir-school