Fix typo in chapter 6.
Fix typo in chapter 6.

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