Fix typo in tutorial_3_one_dimension_sparse.ipynb (#470)

Fixed a typographical error in a code comment.

Author: benjamin0

tutorials/tutorial_3_one_dimension_sparse.ipynb

@@ -317,7 +317,7 @@
    "source": [
     "def direct_fit_predict_constant(X, y, sigma_constant, meas_noise):\n",
     "    # YOUR CODE HERE\n",
-    "    # gamma = sigma_e^{-2} / (sigma_a^{-1} + n sigma_e^{-2})\n",
+    "    # gamma = sigma_e^{-2} / (sigma_a^{-2} + n sigma_e^{-2})\n",
     "    # gamma = \n",
     "    # ratio = sigma_a^2 / sigma_e^2\n",
     "    # ratio = \n",