Documentation: Fix unresponsive link
[Link checking][] failed multiple times for the link to the ACM
server. We now use a CiteSeerX link to the same article, which
hopefully works better.
[Link checking]:
@@ -117,7 +117,7 @@ multiple keys can contribute to each cascading key name.
Cascading is the same as a name resolution and provides a
namespace unification as described in
-[Versatility and Unix semantics in namespace unification](http://dl.acm.org/citation.cfm?id=1138045).
+[Versatility and Unix semantics in namespace unification](http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.81.7888).
Keys without a namespace can not be stored by their nature. So they
are transient: after a restart they are forgotten.
