Modifying the plugins property in XWikiPreferences doesn't reload the plugins

This is because of 2 typos in XWiki.java:

    private static final RegexEntityReference XWIKIPREFERENCE_PROPERTY_REFERENCE = new RegexEntityReference(
        Pattern.compile("plugin"), EntityType.OBJECT_PROPERTY, new RegexEntityReference(
            Pattern.compile(".*:XWiki.XWikiPreference\\[\\d*\\]"), EntityType.OBJECT));

• Typo 1: plugin --> plugins
• Typo 2: XWikiPreference --> XWikiPreferences

Issue is that we didn't have any functional test for this... and we still don't have it... Now since we're phasing out plugins we could hurry that up and get by without writing a functional test for this...