Add compute microversion 2.61 schema in flavors_client

Compute microversion 2.61 adds 'extra_specs' in flavor APIs response
This commit fill the schema gap for that.

Change-Id: Id1f1e51fbae1cc3d0c9efdcd9009f384745915d6
diff --git a/doc/source/microversion_testing.rst b/doc/source/microversion_testing.rst
index ea868ae..3ae03c2 100644
--- a/doc/source/microversion_testing.rst
+++ b/doc/source/microversion_testing.rst
@@ -374,6 +374,10 @@
 
   .. _2.60: https://docs.openstack.org/nova/latest/reference/api-microversion-history.html#maximum-in-queens
 
+  * `2.61`_
+
+  .. _2.61: https://docs.openstack.org/nova/latest/reference/api-microversion-history.html#id55
+
   * `2.63`_
 
   .. _2.63: https://docs.openstack.org/nova/latest/reference/api-microversion-history.html#id57