Fix reference in docs build.

Author: jaraco

docs/using.rst

@@ -8,8 +8,9 @@
 package metadata.  Built in part on Python's import system, this library
 intends to replace similar functionality in the `entry point
 API`_ and `metadata API`_ of ``pkg_resources``.  Along with
-:mod:`importlib.resources` (with new features backported to the
-`importlib_resources`_ package), this can eliminate the need to use the older
+:mod:`importlib.resources` (with new features backported to
+:doc:`importlib_resources <importlib_resources:index>`),
+this package can eliminate the need to use the older
 and less efficient
 ``pkg_resources`` package.