s3: sort providers in docs

CONTRIBUTING.md

@@ -633,6 +633,7 @@ You'll need to modify the following files
 - `docs/content/s3.md`
   - Add the provider at the top of the page.
   - Add a section about the provider linked from there.
+    - Make sure this is in alphabetical order in the `Providers` section.
   - Add a transcript of a trial `rclone config` session
     - Edit the transcript to remove things which might change in subsequent versions
   - **Do not** alter or add to the autogenerated parts of `s3.md`