[BUG][Spec] GitHubReviewers interface is not described

[BorysShulyak]

Describe the bug
GitHubReviewers interface is not described in the DSL Reference.

To Reproduce
Steps to reproduce the behavior:

1. Go to the DSL Reference
2. Go to the GitHubDSL sections.
3. See the requested_reviewers: GitHubReviewers line and try to click on the GitHubReviewers link.
4. See that there is no GitHubReviewers interface description.

Expected behavior
The GitHubReviewers section should be added with the description of GitHubReviewers interface OR the GitHubReviewers reference should be replaced with GitHubUser one.

Screenshots

• https://gyazo.com/8829d172adc101bbc8384427e586da63