Fix heading levels in write_tests doc

This commit fixes the top level header for the write_tests doc.
Previously, the doc title was set as the same as the section. This
caused things like table of contents to confuse the hierarchy of the
sections in the docs. This commit fixes that by setting the doc title
to be a higher lever in the hierarchy.

Change-Id: I2ecb110c766a71e1dc2d9e663ca38ece76dc327d
diff --git a/doc/source/write_tests.rst b/doc/source/write_tests.rst
index 768bf0f..67b55aa 100644
--- a/doc/source/write_tests.rst
+++ b/doc/source/write_tests.rst
@@ -1,7 +1,7 @@
 .. _tempest_test_writing:
 
 Tempest Test Writing Guide
-==========================
+##########################
 
 This guide serves as a starting point for developers working on writing new
 Tempest tests. At a high level tests in Tempest are just tests that conform to