Update the selectThreshold() in Exercise 8

I believe some parameter specifications in selectThreshold() were incorrect due to a copy/paste error. Please take a look.

thanks,
Tigran

Exercise8/exercise8.ipynb

@@ -298,11 +298,10 @@
     "    Parameters\n",
     "    ----------\n",
     "    yval : array_like\n",
-    "        The validation dataset of shape (m x n) where m is the number \n",
-    "        of examples an n is the number of dimensions(features).\n",
+    "        The ground truth labels of shape (m, ).\n",
     "    \n",
     "    pval : array_like\n",
-    "        The ground truth labels of shape (m, ).\n",
+    "        The precomputed vector of probabilities based on mu and sigma2 parameters. It's shape is also (m, ).\n",
     "    \n",
     "    Returns\n",
     "    -------\n",