Update doc version selector link (#2605)

Summary:
The link to version selector has been absolute link, which had been
a trap when reviewing gh-pages deployment from folk.

This commit changes that to relative link.

Pull Request resolved: https://github.com/pytorch/audio/pull/2605

Test Plan:
- https://mthrok.github.io/audio/main/index.html -> click version selector -> https://mthrok.github.io/audio/versions.html
- https://mthrok.github.io/audio/0.12.1/index.html -> click version selector -> https://pytorch.org/audio/versions.html

Reviewed By: carolineechen, nateanl

Differential Revision: D38695645

Pulled By: mthrok

fbshipit-source-id: 91132ac19b8c61f39d304a162435b9c6599ef2b2

docs/source/_templates/layout.html

@@ -2,7 +2,7 @@
 
 {% block sidebartitle %}
     <div class="version">
-      <a href='https://pytorch.org/audio/versions.html'>{{ version }} &#x25BC</a>
+      <a href="{{ pathto('../versions.html', 1) }}">{{ version }} &#x25BC</a>
     </div>
     {% include "searchbox.html" %}
 {% endblock %}