Fix subunit describe calls utility document warnings

This commit fixes subunit describe calls utility document warnings
below.

 WARNING: Definition list ends without a blank line; unexpected
          unindent.

Change-Id: Ife47a85446f49cd0de6bf7af3381d1a28e4311d0
diff --git a/tempest/cmd/subunit_describe_calls.py b/tempest/cmd/subunit_describe_calls.py
index c990add..da7f426 100644
--- a/tempest/cmd/subunit_describe_calls.py
+++ b/tempest/cmd/subunit_describe_calls.py
@@ -45,6 +45,7 @@
 
 Ports file JSON structure
 ^^^^^^^^^^^^^^^^^^^^^^^^^
+::
 
   {
       "<port number>": "<name of service>",
@@ -54,6 +55,8 @@
 
 Output file JSON structure
 ^^^^^^^^^^^^^^^^^^^^^^^^^^
+::
+
   {
       "full_test_name[with_id_and_tags]": [
           {