Fix class reference.

docs/examples/example_propensity.ipynb

@@ -89,7 +89,7 @@
     "dataset, we just use it for illustrational purposes.\n",
     "```\n",
     "\n",
-    "Now we can use a custom ``sklearn``-like classifier: :class:`~metalearners.utils.FixedBinaryPropensity`.\n",
+    "Now we can use a custom ``sklearn``-like classifier: {class}`~metalearners.utils.FixedBinaryPropensity`.\n",
     "The latter can be used like any ``sklearn`` classifier but will always return the same propensity,\n",
     "independently of the observed covariates. This propensity has to be provided at initialization via the\n",
     "``propensity_score`` parameter."