docs: fix problems with contributors documentation

If reader used another languages and click to any "contributor" pages him received 404 error.

Problem with redirects in openstack-manuals:
0d13b52d1c/www/.htaccess (L191)

After MR:
4aed59d263

For fix this problem, we renamed directory "contributor" to "contributors".

Thanks to Mikhail Danilkin for the information about this error.

Change-Id: I10a746fa258de0eb81f72124b2a9466e67f71bdd

doc/source/index.rst

@@ -21,8 +21,8 @@ OpenStack-Ansible 2025.1 (Epoxy) is currently Under Development.
    Release notes <https://docs.openstack.org/releasenotes/openstack-ansible/unreleased.html>
    admin/index
    user/index
-   contributor/contributing
-   contributor/index
+   contributors/contributing
+   contributors/index
    reference/index
 
 2024.2 (Dalmatian): Maintained