Porter's Handbook - Fix link in Upgrading chapter

Pull Request: #250

documentation/content/en/books/porters-handbook/upgrading/_index.adoc

@@ -51,7 +51,7 @@ When a port is not the most recent version available from the authors, update th
 The port might have already been updated to the new version.
 
 When working with more than a few ports, it will probably be easier to use Git to keep the whole ports collection up-to-date,
-as described in the extref:{handbook}[Handbook, ports-using].
+as described in the extref:{handbook}ports[Using the Ports Collection, ports-using].
 This will have the added benefit of tracking all the port's dependencies.
 
 The next step is to see if there is an update already pending.