Test fix for link to nbviewer. #238

Trying to make a relative link to the table of contents so browsing
works both locally and remotely.

00-Preface.ipynb

@@ -4,7 +4,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "[Table of Contents](http://nbviewer.ipython.org/github/rlabbe/Kalman-and-Bayesian-Filters-in-Python/blob/master/table_of_contents.ipynb)"
+    "[Table of Contents](./table_of_contents.ipynb)"
    ]
   },
   {
@@ -1135,7 +1135,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.6.3"
+   "version": "3.6.4"
   }
  },
  "nbformat": 4,