doc: Add back target for datapath testing section.
This one really is used.  I'm honestly not sure how I missed this, because
I did test things.
Fixes: ("doc: Remove some link targets that aren't used anywhere.")
Reported-by: Andy Zhou
Acked-by: Andy Zhou
@@ -205,6 +205,8 @@ omit ``RYUDIR``
of Open vSwitch and Ryu in your bug report, plus any other information
needed to reproduce the problem.
+.. _datapath-testing:
+
Datapath testing
~~~~~~~~~~~~~~~~
