Remove unnecessary indentations in releasenotes index

This commit removes unnecessary indentations in releasenotes
index.rst. An extra indentation is rendered as a quote which is
weird.

Change-Id: I2c33c7870946622c3c76717c8a621f0663ce64e1
diff --git a/releasenotes/source/index.rst b/releasenotes/source/index.rst
index d301f9c..2518703 100644
--- a/releasenotes/source/index.rst
+++ b/releasenotes/source/index.rst
@@ -2,22 +2,22 @@
  Tempest Release Notes
 ===========================
 
- .. toctree::
-    :maxdepth: 1
+.. toctree::
+   :maxdepth: 1
 
-    unreleased
-    v18.0.0
-    v17.2.0
-    v17.1.0
-    v17.0.0
-    v16.1.0
-    v16.0.0
-    v15.0.0
-    v14.0.0
-    v13.0.0
-    v12.0.0
-    v11.0.0
-    v10.0.0
+   unreleased
+   v18.0.0
+   v17.2.0
+   v17.1.0
+   v17.0.0
+   v16.1.0
+   v16.0.0
+   v15.0.0
+   v14.0.0
+   v13.0.0
+   v12.0.0
+   v11.0.0
+   v10.0.0
 
 Indices and tables
 ==================