docs: Fix incorrect docstrings in reStructuredText format

Several docstrings have incorrect format, what prevents to generate
a documentation using sphinx-apidoc extension.

Change-Id: I66431c7030089173d7635c1e2a72765db21f9780
diff --git a/doc/source/data/tempest-plugins-registry.header b/doc/source/data/tempest-plugins-registry.header
index 0de12b7..831d8a6 100644
--- a/doc/source/data/tempest-plugins-registry.header
+++ b/doc/source/data/tempest-plugins-registry.header
@@ -3,9 +3,9 @@
   job.  You should edit the files data/tempest-plugins-registry.footer
   and data/tempest-plugins-registry.header instead of this one.
 
-==========================
- Tempest Plugin Registry
-==========================
+=======================
+Tempest Plugin Registry
+=======================
 
 Since we've created the external plugin mechanism, it's gotten used by
 a lot of projects. The following is a list of plugins that currently