Fix to the error: unknown option points-at when invoking make docs.

Before the fix:
elixir$ make docs
==> elixir (compile)
error: unknown option `points-at'
usage: git tag ...


After the fix:
elixir$ make docs
==> elixir (compile)
mkdir -p ebin
rm -rf docs
cp -R -f lib/*/ebin/*.beam ./ebin
bin/elixir ../ex_doc/bin/ex_doc "Elixir" "0.12.1-dev" "./ebin" -m Kernel -u "https://github.com/elixir-lang/elixir" --source-ref "349873ab633ae6a2646674073de90e808749c85c"
rm -rf ebin
elixir$