Update OS_AUTH_URL in Configuration.rst

I am a new participant, and when I read the document I found that the command in the OS_AUTH_URL example is v2.0, so I want to update it.

Change-Id: I973adc303a3cb37ce377ca4e31d1d666cd41b358
diff --git a/doc/source/configuration.rst b/doc/source/configuration.rst
index 064bf51..c4834b7 100644
--- a/doc/source/configuration.rst
+++ b/doc/source/configuration.rst
@@ -136,7 +136,7 @@
 
     ::
 
-        OS_AUTH_URL=http://$SERVICE_HOST:5000/v2.0
+        OS_AUTH_URL=http://$SERVICE_HOST:5000/v3.0
 
 KEYSTONECLIENT\_DEBUG, NOVACLIENT\_DEBUG
     Set command-line client log level to ``DEBUG``. These are commented