Add tag_name to update_release documentation
Add tag_name to update_release documentation

The [edit_release](https://developer.github.com/v3/repos/releases/#edit-a-release) endpoint accepts a tag_name so that should be documented here.

Furthermore, when using the `update_release` method without specifying a tag_name, the tag in Github is overwritten. 

For example, my tag was "v4.0" and after calling `update_release` without a tag name it became "untagged-{commit-hash}".
pvdb/octokit.rb,JuanitoFatas/octokit.rb,pvdb/octokit.rb,octokit/octokit.rb,JuanitoFatas/octokit.rb,octokit/octokit.rb