Update Output Widget.ipynb (#3881)

Fix: the `itertools` library has not been loaded before being used.

Author: tomichec

docs/source/examples/Output Widget.ipynb

@@ -354,6 +354,7 @@
     "```python\n",
     "import threading\n",
     "import time\n",
+    "import itertools\n",
     "\n",
     "def run():\n",
     "    for i in itertools.count(0):\n",