Fix the spec and docs for Integer.undigits/2
Fix the spec and docs for Integer.undigits/2

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