Update call_center.ipynb

solutions/object_oriented_design/call_center/call_center.ipynb

@@ -158,7 +158,7 @@
     "\n",
     "    def dispatch_call(self, call):\n",
     "        if call.rank not in (Rank.OPERATOR, Rank.SUPERVISOR, Rank.DIRECTOR):\n",
-    "            raise ValueError('Invalid call rank: {}'.format(call.rank))\n",
+    "            raise ValueError(f'Invalid call rank: {call.rank}')\n",
     "        employee = None\n",
     "        if call.rank == Rank.OPERATOR:\n",
     "            employee = self._dispatch_call(call, self.operators)\n",