[DOC] Fix "Title level inconsistent" warning

This commit fixes the warning "Title level inconsistent". And the module
API docs should be shown up.

Change-Id: Ie9a3656b0a58300d611352d054868440f0ffd387
diff --git a/doc/source/library/credential_providers.rst b/doc/source/library/credential_providers.rst
index f4eb37d..d96c97a 100644
--- a/doc/source/library/credential_providers.rst
+++ b/doc/source/library/credential_providers.rst
@@ -130,19 +130,18 @@
   # role
   provider.clear_creds()
 
-API Reference
-=============
 
-------------------------------
+API Reference
+-------------
+
 The dynamic credentials module
-------------------------------
+''''''''''''''''''''''''''''''
 
 .. automodule:: tempest.lib.common.dynamic_creds
    :members:
 
---------------------------------------
 The pre-provisioned credentials module
---------------------------------------
+''''''''''''''''''''''''''''''''''''''
 
 .. automodule:: tempest.lib.common.preprov_creds
    :members: