Fix `responsible to keep` → `responsible for keeping` in env.ex doc
Fix `responsible to keep` → `responsible for keeping` in env.ex doc

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