Update microversion doc for compute microversion 2.3

compute microversion 2.3 schema is already covered in
I2f36a51fa47b28df1afada5396dec3ce7dc43d78

This commit update the doc for the same.

Change-Id: I58160cc2bd42cabce1abc482dd769e2323c90026
diff --git a/doc/source/microversion_testing.rst b/doc/source/microversion_testing.rst
index c1981f9..06062c2 100644
--- a/doc/source/microversion_testing.rst
+++ b/doc/source/microversion_testing.rst
@@ -302,6 +302,10 @@
 
   .. _2.2: https://docs.openstack.org/nova/latest/reference/api-microversion-history.html#id2
 
+  * `2.3`_
+
+  .. _2.3: http://docs.openstack.org/developer/nova/api_microversion_history.html#maximum-in-kilo
+
   * `2.6`_
 
   .. _2.6: https://docs.openstack.org/nova/latest/reference/api-microversion-history.html#id5