Clarify blank line rule in newlines_styleguide.md

## What does this MR do?

* Corrects a typo in the GitLab Ruby newlines style guide
* Makes some vague language in the original guideline more precise

## Are there points in the code the reviewer needs to double check?

n/a

## Why was this MR needed?

* There was a typo in the original text.
* The original text was vague.

## What are the relevant issue numbers?

n/a

## Screenshots (if relevant)

n/a

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added  (Is this recommended for this kind of patch?)
- ~~[Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)~~
- ~~API support added~~
- Tests
  - ~~Added for this feature/bug~~
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !6048