Fix bar x

code/chap03ex.ipynb

@@ -708,12 +708,12 @@
     "the elements and dividing by n.  If you are given a PMF, you can\n",
     "still compute the mean, but the process is slightly different:\n",
     "%\n",
-    "$$ \bar x = \\sum_i p_i~x_i $$\n",
+    "$$ \\bar x = \\sum_i p_i~x_i $$\n",
     "%\n",
     "where the $x_i$ are the unique values in the PMF and $p_i=PMF(x_i)$.\n",
     "Similarly, you can compute variance like this:\n",
     "%\n",
-    "$$ S^2 = \\sum_i p_i~(x_i - \bar x)^2 $$\n",
+    "$$ S^2 = \\sum_i p_i~(x_i - \\bar x)^2 $$\n",
     "% \n",
     "Write functions called `PmfMean` and `PmfVar` that take a\n",
     "Pmf object and compute the mean and variance.  To test these methods,\n",