Fix wording in call center solution (#197)

solutions/object_oriented_design/call_center/call_center.ipynb

@@ -24,9 +24,9 @@
     "    * Operator, supervisor, director\n",
     "* Can we assume operators always get the initial calls?\n",
     "    * Yes\n",
-    "* If there is no free operators or the operator can't handle the call, does the call go to the supervisors?\n",
+    "* If there is no available operators or the operator can't handle the call, does the call go to the supervisors?\n",
     "    * Yes\n",
-    "* If there is no free supervisors or the supervisor can't handle the call, does the call go to the directors?\n",
+    "* If there is no available supervisors or the supervisor can't handle the call, does the call go to the directors?\n",
     "    * Yes\n",
     "* Can we assume the directors can handle all calls?\n",
     "    * Yes\n",