fix: Fix url of Floodlight

The url of Floodlight[1] is unavailable.

Update it to an accessible project url[2].

[1] http://www.projectfloodlight.org/floodlight/
[2] https://github.com/floodlight/floodlight

Change-Id: I8de0cb952e0fa3b9a12fc27ab2764b1552524128

Author: Wei Lingfei

doc/source/admin/archives/config-agents.rst

@@ -179,7 +179,7 @@ functionality. However, the following plug-ins already have built-in L3
 capabilities:
 
 -  Big Switch/Floodlight plug-in, which supports both the open source
-   `Floodlight <http://www.projectfloodlight.org/floodlight/>`__
+   `Floodlight <https://github.com/floodlight/floodlight>`__
    controller and the proprietary Big Switch controller.
 
    .. note::