Sync Sphinx requirement

1. Sync sphinx dependency with global requirements. It caps python 2 since
sphinx 2.0 no longer supports Python 2.7.
2. Remove unncessary "=="

Change-Id: I6988ed388cc6543bc49e62172aa93f34fdca8e95
diff --git a/releasenotes/source/index.rst b/releasenotes/source/index.rst
index 1788b85..0766931 100644
--- a/releasenotes/source/index.rst
+++ b/releasenotes/source/index.rst
@@ -1,6 +1,6 @@
-============================================
- octavia_tempest_plugin Release Notes
-============================================
+====================================
+octavia_tempest_plugin Release Notes
+====================================
 
 .. toctree::
    :maxdepth: 1