Fixes broken link issue in #54660

Updated tf2up.ml with Github link as per author's request.

Author: mohantym

site/en/guide/migrate/upgrade.ipynb

@@ -580,7 +580,7 @@
         "\n",
         "- This script does not reorder arguments. Instead, the script adds keyword arguments to functions that have their arguments reordered.\n",
         "\n",
-        "- Check out [tf2up.ml](http://tf2up.ml) for a convenient tool to upgrade Jupyter\n",
+        "- Check out [tf2up.ml](https://github.com/lc0/tf2up) for a convenient tool to upgrade Jupyter\n",
         "  notebooks and Python files in a GitHub repository.\n",
         "\n",
         "To report upgrade script bugs or make feature requests, please file an issue on [GitHub](https://github.com/tensorflow/tensorflow/issues)."
@@ -600,4 +600,4 @@
   },
   "nbformat": 4,
   "nbformat_minor": 0
-}
+}