add target to custom distribution notebook

Author: OriolAbril

examples/pymc3_howto/custom_distribution.ipynb

@@ -4,6 +4,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
+    "(custom_distribution)=\n",
     "# Defining a Custom Distribution in PyMC3"
    ]
   },
@@ -534,7 +535,7 @@
  ],
  "metadata": {
   "kernelspec": {
-   "display_name": "Python 3",
+   "display_name": "Python 3 (ipykernel)",
    "language": "python",
    "name": "python3"
   },
@@ -548,7 +549,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.8.1"
+   "version": "3.9.7"
   }
  },
  "nbformat": 4,