Fix the example of testr testing

Closes-Bug: #1263885

By separating the negative tests, the README example has not been valid.
This commit fix this.

Change-Id: I299618bb9da63b47af17efbe4ea226ebf770f86a
diff --git a/README.rst b/README.rst
index 96f6e4c..4098e32 100644
--- a/README.rst
+++ b/README.rst
@@ -67,7 +67,7 @@
 
 To run one single test  ::
 
-    $> testr run --parallel tempest.api.compute.servers.test_server_actions.ServerActionsTestJSON.test_rebuild_nonexistent_server
+    $> testr run --parallel tempest.api.compute.servers.test_servers_negative.ServersNegativeTestJSON.test_reboot_non_existent_server
 
 Alternatively, you can use the run_tests.sh script which will create a venv
 and run the tests or use tox to do the same.