[MRG+1] Fix "cite us" link in sidebar (scikit-learn#8142)

* Fix citation link in sidebar

* Improve formatting of 'cite us' link

doc/themes/scikit-learn/layout.html

@@ -275,7 +275,7 @@ <h2>Machine Learning in Python</h2>
  <p class="doc-version"><b>{{project}} v{{ release|e }}</b><br/>
  <a href="http:https://scikit-learn.org/stable/support.html#documentation-resources">Other versions</a></p>
  {%- endif %}
- <p class="citing">Please <b><a href="about.html#citing-scikit-learn" style="font-size: 110%;">cite us </a></b>if you use the software.</p>
+ <p class="citing">Please <b><a href="{{ pathto('about').replace('#', '') }}#citing-scikit-learn" style="font-size: 110%;">cite us </a></b>if you use the software.</p>
  {{ toc }}
  </div>
 </div>