Update README to use stestr instead testr

Tempest uses stestr since Iff4abef50178bdc83b868eed4a906e22d790762b was
merged. This patch chaned README to fix instructions for a default
configuration

Change-Id: I90860c4b8e3a0cfdee780fc587d9af21b1f36956
diff --git a/README.rst b/README.rst
index d0ca6e1..126027c 100644
--- a/README.rst
+++ b/README.rst
@@ -21,4 +21,4 @@
 
 In tempest directory, run::
 
-  testr run tempest_horizon.tests.scenario.test_dashboard_basic_ops.TestDashboardBasicOps
+  stestr run tempest_horizon.tests.scenario.test_dashboard_basic_ops.TestDashboardBasicOps