added text description for TOC link

Chapter04_Gaussians/Gaussians.ipynb

@@ -1,7 +1,7 @@
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@@ -12,7 +12,7 @@
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      "source": [
-      "http://nbviewer.ipython.org/github/rlabbe/Kalman-and-Bayesian-Filters-in-Python/blob/master/table_of_contents.ipynb"
+      "[Table of Contents](http://nbviewer.ipython.org/github/rlabbe/Kalman-and-Bayesian-Filters-in-Python/blob/master/table_of_contents.ipynb)"
      ]
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