Parameter for filtering repository issues by milestone is called milestone

The parameter for filtering repository issues by milestone is called `milestone`, not `filter`. I believe this was unintentionally changed in [this commit here](https://github.com/github/developer.github.com/commit/3aa7c72d9b667fd74a1ce1ff67990aab26906750#diff-8bbb992226a6e795d4477570fd73cbb9R53).

cc @gjtorikian 

Thanks for the report @leandro-lucarella-sociomantic!