Use proper formatting in the docs for System.pid/0
Use proper formatting in the docs for System.pid/0

[ci skip]

pedrosnk/elixir,ggcampinho/elixir,lexmag/elixir,lexmag/elixir,joshprice/elixir,michalmuskala/elixir,kimshrier/elixir,kimshrier/elixir,kelvinst/elixir,elixir-lang/elixir,ggcampinho/elixir,pedrosnk/elixir,kelvinst/elixir