Update CONTRIBUTING.md
Update CONTRIBUTING.md

renamed 'issues tracker' to 'issue tracker' for consistency
kimshrier/elixir,pedrosnk/elixir,elixir-lang/elixir,antipax/elixir,gfvcastro/elixir,ggcampinho/elixir,ggcampinho/elixir,beedub/elixir,beedub/elixir,lexmag/elixir,kelvinst/elixir,kimshrier/elixir,gfvcastro/elixir,lexmag/elixir,antipax/elixir,joshprice/elixir,michalmuskala/elixir,kelvinst/elixir,pedrosnk/elixir