DOC: Don't link to index.rst

... because this is converted to a link to index.html, which wasn't the
idea here.

Author: mgeier

doc/usage.ipynb

@@ -42,7 +42,7 @@
     "For an example, see this project's  [conf.py](conf.py) file.\n",
     "\n",
     "Once your `conf.py` is in place, edit the file named `index.rst` and add the file names of your notebooks (without the `.ipynb` extension) to the [toctree](http://www.sphinx-doc.org/en/master/usage/restructuredtext/directives.html#directive-toctree) directive.\n",
-    "For an example, see this project's  [index.rst](index.rst) file."
+    "For an example, see this project's `doc/index.rst` file."
    ]
   },
   {