Fix the aria-expanded status toggle in the sidebar menu fo a directy accessed subpage

[rianrietveld]

From the accessibility audit, issue #232

SC 4.1.2 Name, role, value: When directly visiting a sub-page, e.g. https://wpaccessibility.org/docs/topics/content/links/link-image/, the sub-menus are expanded by default to show the current page in the menu. In this case the buttons that expand and collapse the sub-menu have a wrong value of "false" in the aria-expanded attribute. They are expanded, but they get announced as "collapsed".

The work:
Fix the aria-expanded status toggle.