CLI doc missing colon for code block

bug 1194360

Added double colon to indicate a code block for the bottom
of this page:
http://docs.openstack.org/developer/tempest/field_guide/cli.html

Change-Id: Icb555adee6716071fa7afb58ee2f83e7a745a585
diff --git a/tempest/cli/README.rst b/tempest/cli/README.rst
index 76b05a3..3eae492 100644
--- a/tempest/cli/README.rst
+++ b/tempest/cli/README.rst
@@ -36,7 +36,7 @@
 If a test is validating the cli for bad data, it should do it with
 assertRaises.
 
-A reasonable example of an existing test is as follows:
+A reasonable example of an existing test is as follows::
 
     def test_admin_list(self):
         self.nova('list')