Change reST targets in setup.cfg spec.

I chose “setupcfg” as prefix instead of “packaging-setupcfg” because the scope
of the spec is not limited to packaging: it’s intended as a language-agnostic
document for packaging tools developers as well as Python authors.
diff --git a/Doc/packaging/commandhooks.rst b/Doc/packaging/commandhooks.rst
index fd33357..0a3d044 100644
--- a/Doc/packaging/commandhooks.rst
+++ b/Doc/packaging/commandhooks.rst
@@ -11,7 +11,7 @@
 processed), but before it is run. The post-hooks are run after the command
 itself. Both types of hooks receive an instance of the command object.
 
-See also global setup hooks in :ref:`packaging-setupcfg`.
+See also global setup hooks in :ref:`setupcfg-spec`.
 
 
 Sample usage of hooks