Minor doc change ERL_COMPILER_OPTS to ERL_COMPILER_OPTIONS

[rambocoder]

In these two files, ERL_COMPILER_OPTS is still mentioned instead of ERL_COMPILER_OPTIONS

https://github.com/elixir-lang/elixir/blob/master/bin/elixirc#L14
https://github.com/elixir-lang/elixir/blob/master/bin/elixirc.bat#L24