corrected mistake in PF formula

Author: pkck28

books/const_seq_sampling/prob_feas.ipynb

@@ -18,7 +18,7 @@
     "\n",
     "$$\n",
     "    \\begin{align*}\n",
-    "        PF_i(x) = P[G_i(x) \\leq 0] = \\Phi \\Bigg( -\\frac{\\hat{g}_i(x)}{\\hat{\\sigma}_{g,i}(x)} \\leq 0 \\Bigg),\n",
+    "        PF_i(x) = P[G_i(x) \\leq 0] = \\Phi \\Bigg( -\\frac{\\hat{g}_i(x)}{\\hat{\\sigma}_{g,i}(x)}\\Bigg),\n",
     "    \\end{align*}\n",
     "$$\n",
     "\n",