Fix README

The oslo config-generator-tempest.conf location is defined
as still being:

'etc/config-generator.tempest.conf'

But after the changes made in tempest-run-cmd bp the location
changed to:

'tempest/cmd/config-generator.tempest.conf'.

Change-Id: I53b0ab18a217eb186c51eb5a2e35489967fc5c55
Closes-Bug: #1606586
diff --git a/README.rst b/README.rst
index d109a2d..13f4f61 100644
--- a/README.rst
+++ b/README.rst
@@ -223,7 +223,7 @@
 
     $ cd $TEMPEST_ROOT_DIR
     $ oslo-config-generator --config-file \
-        etc/config-generator.tempest.conf \
+        tempest/cmd/config-generator.tempest.conf \
         --output-file etc/tempest.conf
 
 After that, open up the ``etc/tempest.conf`` file and edit the