Fix the changelog for the Fiber's guard page.

[nemanja-boric-sociomantic]

During the review of GitHub dlang/druntime#1698, the "END OF FIBER" marker data
was removed to avoid dirtying the guard page. Hence, the corresponding
comment in the release notes doesn't apply anymore.

See dlang/druntime#1821

[dlang-bot]

Thanks for your pull request, @nemanja-boric-sociomantic! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.

Some tips to help speed things up:

• smaller, focused PRs are easier to review than big ones

• try not to mix up refactoring or style changes with bug fixes or feature enhancements

• provide helpful commit messages explaining the rationale behind each change

Bear in mind that large or tricky changes may require multiple rounds of review and revision.

Please see CONTRIBUTING.md for more information.

Bugzilla references

Your PR doesn't reference any Bugzilla issue.

If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.