docs: clarify that the APK can only be built

Right now, the app's release page doesn't provide APKs as
artifacts. So, correct the sentence to clarify that the APKs
could only be built for previous versions of the app.

Fixes commons-app#56

Author: kaartic-sn-20010

docs.html

@@ -85,7 +85,7 @@ <h2 id="app-versions"><a href="#app-versions">App versions</a></h2>
             <li>Beta if the version string contains two dots. Example: 3.0.1</li>
             <li>Alpha if the version string contains three dots. Example: 3.0.1.1019</li>
         </ol>
-        <p>If you want to switch back to a previous version, please <a href="https://github.com/commons-app/apps-android-commons/releases">download the APK of the version you want</a> and install it (you may need to uninstall first).</p>
+        <p>If you want to switch back to a previous version, please <a href="https://github.com/commons-app/apps-android-commons/releases">download the source of the corresponding version</a> and <a href="https://github.com/commons-app/commons-app-documentation/blob/master/android/Quick-start-guide-for-Developers.md#building-the-source-code">build the 'prod' flavor of the APK</a> and install it (you may need to uninstall first).</p>
 
         <h2 id="getting-app-logs-from-android-studio"><a href="#getting-app-logs-from-android-studio">Getting app logs from Android Studio</a></h2>
         <p>Brief walkthrough for users on how to post logs if they are encountering issues.</p>