[web-php] PR #501: Fix content layout on pages without sidebars for 768-979px viewport widths

Pull Request: https://github.com/php/web-php/pull/501 Author: SteveDesmond-ca Per #500, pages without sidebar content (in an aside tag) are still pushed to the side at 70% width. I believe this is due to an over-eager selector (the one line changed in this PR). The combination of #layout-content and :only-child is used in multiple places elsewhere in the site's CSS files, so I figured that was the safest option to minimize the amount being changed.