Update p-median.ipynb

fixes #466
A typo is fixed

notebooks/p-median.ipynb

@@ -25,7 +25,7 @@
     "            & Y_{j} \\in \\{0,1\\} & \\forall j \\in J && (6) \\\\ \\end{array}$\n",
     "\n",
     "> $\\begin{array} \\displaystyle \\textbf{Where:}\\\\ & & \\displaystyle i & \\small = & \\textrm{index referencing nodes of the network as demand} \\\\\n",
-    "& & j & \\small = & \\textrm{index referencing nodes of the network as potential facility sites
+    "& & j & \\small = & \\textrm{index referencing nodes of the network as potential facility sites} \\\\\n",
     "& & d_{ij} & \\small = & \\textrm{shortest distance or travel time between nodes i and j} i \\textrm{ and } j \\\\\n",
     "& & p & \\small = & \\textrm{number of facilities to be located} \\\\\n",
     "& & a_i & \\small = & \\textrm{service load or population demand at } i \\\\\n",