docs: use site.time

Author: kou

docs/_config.yml

@@ -3,7 +3,6 @@ markdown: kramdown
 highlighter: rouge
 version: 1.2.0
 release_date: 2022-04-11
-copyright_year: 2017-2022
 exclude:
   - "Rakefile"
   - "Gemfile"

docs/_layouts/base.html

@@ -48,7 +48,7 @@
       <hr>
       <div class="container">
         {% include contribute.html %}
-        <p class="copyright">&copy; {{ site.copyright_year }} <a href="https://github.com/clear-code/xmlua/graphs/contributors">XMLua developers</a></p>
+        <p class="copyright">&copy; 2017-{{ site.time | date: "%Y"}} <a href="https://github.com/clear-code/xmlua/graphs/contributors">XMLua developers</a></p>
       </div>
     </footer>