Fix directory tree appearance in document

This commit fixes the directory tree appearance in document. It looks
like that the original rst line blocks don't work in the current
renderer. As a result, the directory structure is a little difficult to
understand. So, this commit use a literal block instead of that as a
quick fix.

Change-Id: I897cec40cc1ee5ead50997e75c474e1f78ee763e
diff --git a/tempest/README.rst b/tempest/README.rst
index 663653e..62821de 100644
--- a/tempest/README.rst
+++ b/tempest/README.rst
@@ -12,10 +12,12 @@
 and guidelines. Below is the overview of the Tempest respository structure
 to make this clear.
 
-| tempest/
-|    api/ - API tests
-|    scenario/ - complex scenario tests
-|    tests/ - unit tests for Tempest internals
+ .. code-block:: console
+
+    tempest/
+       api/ - API tests
+       scenario/ - complex scenario tests
+       tests/ - unit tests for Tempest internals
 
 Each of these directories contains different types of tests. What
 belongs in each directory, the rules and examples for good tests, are