Fix @spec sort_by
Fix @spec sort_by

Use format provided by José Valim that uses `when` to eliminate the need
for `@type mapped_element`.

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