Fix the old or invalid doc links

The urls in docs are old or invalid.
Replace them with the right ones for the docs according to
the official OpenStack websites.

Change-Id: I259702f5771e975fe24f7e97a450f342d8ad2beb
diff --git a/CONTRIBUTING.rst b/CONTRIBUTING.rst
index a4755cc..30628c0 100644
--- a/CONTRIBUTING.rst
+++ b/CONTRIBUTING.rst
@@ -1,14 +1,14 @@
 If you would like to contribute to the development of OpenStack, you must
 follow the steps in this page:
 
-   http://docs.openstack.org/infra/manual/developers.html
+   https://docs.openstack.org/infra/manual/developers.html
 
 If you already have a good understanding of how the system works and your
 OpenStack accounts are set up, you can skip to the development workflow
 section of this documentation to learn how changes to OpenStack should be
 submitted for review via the Gerrit tool:
 
-   http://docs.openstack.org/infra/manual/developers.html#development-workflow
+   https://docs.openstack.org/infra/manual/developers.html#development-workflow
 
 Pull requests submitted through GitHub will be ignored.