doc : add reference to the wikipedia page

Author: beachdweller

10_root_finding/40_fixed_point_iteration.ipynb

@@ -388,6 +388,10 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
+    "# Reference\n",
+    "\n",
+    "* \"Fixed-point iteration.\" Wikipedia, The Free Encyclopedia. Wikimedia Foundation, Inc. Last edited 4 June 2025. https://en.wikipedia.org/wiki/Fixed-point_iteration\n",
+    "\n",
     "# Acknowledgments\n",
     "\n",
     "This work was supported by assistance from Grok 3, created by xAI.\n",