Fix up minor problems in help topics

Please wait until #3041924: [META] Convert hook_help() module overview text to topics is otherwise fixed before making patches for this issue. It is currently Postponed.

We have made a few typos and there may be other minor problems with help topics to fix up... This issue is to collect them.

Here is the list of fixes to do:

1. benjifisher found this typo: in system.config_error.html.twig:
   

   enter the URL for your 403/403 pages, starting 
   

   Should say 403/404.

2. alexpott found this in contact.setting_default.html.twig
   

   <p>{% trans %}Set a site-wide contact form to be the default contact form (the form that is shown on the <em>/contact</em> URL).{% endtrans %}</p>
   

   It's possible the /contact path is altered so might not be on /contact. I think we should link to the site wide contact form using the contact.site_page route.

3. On #3150364: Add a description for the language toolbar button to the CKEditor help page some text is being added to the CKEditor hook_help about the Language button that can be added to the toolbar. We need to make sure that information is also in our CKEditor topic or in an appropriate multilingual topic, or both, once that issue is finalized.
4. On #3150364: Add a description for the language toolbar button to the CKEditor help page they also made a list of accessibility features and a section about making accessible text. I'm not sure we covered that well in our topic on Accessibility, so we should check. See this screenshot of the output of one of the patches: