Optimize the link address

Use https instead of http to ensure the safety

Change-Id: I4b8be0de1331450753508517558acdcca66e94c5
diff --git a/CONTRIBUTING.rst b/CONTRIBUTING.rst
index 595730a..243a9c0 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.