Fix Regex specs

A couple of errors in the specs in the `Regex` module were found by the dialyzer warnings in #1569. A warning was:

```
ib/elixir/lib/regex.ex:69: Invalid type specification for function 'Elixir.Regex':compile/2. The success typing is (_,binary() | [any()]) -> {'error',{'invalid_option' | string(),binary() | non_neg_integer()}} | {'ok',{'Elixir.Regex',{'re_pattern',_,_,_},binary(),binary(),'nil' | [atom()]}}
```