Update README to use stestr instead of testr for unit tests

This commit updates README.rst to use stestr instead of testr for unit
tests. The patch[1] for changing to use stestr was already merged. It's
good to change the document to use stestr instead of testr for the unit
test section at least.

[1] Iff4abef50178bdc83b868eed4a906e22d790762b

Change-Id: Ib6485d8281aaa5991188ea5c1ba9a6a4c827e8a6
diff --git a/README.rst b/README.rst
index 17d4cba..c67362a 100644
--- a/README.rst
+++ b/README.rst
@@ -183,11 +183,11 @@
 Tempest also has a set of unit tests which test the Tempest code itself. These
 tests can be run by specifying the test discovery path::
 
-    $ OS_TEST_PATH=./tempest/tests testr run --parallel
+    $ stestr --test-path ./tempest/tests run
 
-By setting OS_TEST_PATH to ./tempest/tests it specifies that test discover
-should only be run on the unit test directory. The default value of OS_TEST_PATH
-is OS_TEST_PATH=./tempest/test_discover which will only run test discover on the
+By setting ``--test-path`` option to ./tempest/tests it specifies that test discover
+should only be run on the unit test directory. The default value of ``test_path``
+is ``test_path=./tempest/test_discover`` which will only run test discover on the
 Tempest suite.
 
 Alternatively, there are the py27 and py35 tox jobs which will run the unit