use a more common rst header hiearchy

While rst doesn't actually care about the order of headers, reviewers
sometimes do. And the build in emacs mode has a certain order
specified that it can easily rotate between.

Standardize on == h1, = h2, - h3, ~ h4 in the code.

Change-Id: I80ff6df6ef0703a3c3005809069428018bb355d4
diff --git a/doc/source/plugins.rst b/doc/source/plugins.rst
index b4136c4..485cd0f 100644
--- a/doc/source/plugins.rst
+++ b/doc/source/plugins.rst
@@ -6,7 +6,7 @@
 support for additional projects and features.
 
 Extras.d Hooks
-~~~~~~~~~~~~~~
+==============
 
 These hooks are an extension of the service calls in
 ``stack.sh`` at specific points in its run, plus ``unstack.sh`` and
@@ -93,7 +93,7 @@
    but after ``unstack.sh`` has been called.
 
 Hypervisor
-~~~~~~~~~~
+==========
 
 Hypervisor plugins are fairly new and condense most hypervisor
 configuration into one place.