Fix link to Kernel.<=/2 in Enum.sort_by/3 @doc
Fix link to Kernel.<=/2 in Enum.sort_by/3 @doc

When I first wrote the docs, I didn't know I had to include the
`Kernel.` to make the automatic linking work.  I double checked after
reading the documentation guide for Elixir 1.2 and noticed that the
`<=/2` wasn't linking.

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