Fix volume microversion link in doc

Current link is of ocata, we should have latest doc
link for better maintenance.

Change-Id: Ic8194bb1e4263324267852fccd4f9f9a029ce86a
diff --git a/doc/source/microversion_testing.rst b/doc/source/microversion_testing.rst
index 573b7d4..307eb07 100644
--- a/doc/source/microversion_testing.rst
+++ b/doc/source/microversion_testing.rst
@@ -338,4 +338,4 @@
 
  * `3.3`_
 
- .. _3.3:  https://docs.openstack.org/cinder/ocata/devref/api_microversion_history.html#id4
+ .. _3.3:  https://docs.openstack.org/cinder/latest/contributor/api_microversion_history.html#id4