Replace any link to api.drupal.org with links to docs.backdropcms.org

[avpaderno]

In the ajax_example/ajax_example_progressbar.inc file, there is a comment with a link to api.drupal.org.

/**
 * Implements hook_FORMID().
 *
 * Build a landing-page form for the progress bar example.
 *
 * @see https://api.drupal.org/api/drupal/developer%21topics%21forms_api_reference.html/7#ajax_progress
 */

That link, and other links for api.drupal.org, needs to be corrected with a link to docs.backdropcms.org.

[avpaderno]

I did not wait a week to commit this change because it is about a single code line.

Links like these could need to be changed, depending on what the solution for backdrop-ops/docs.backdropcms.org#228 will be.