feat: Add post-tutorial revisions (#3)

* Show upperlimit API

book/HelloWorld.ipynb

@@ -689,6 +689,26 @@
     "print(f\"Upper limit (obs): μ = {np.interp(0.05, observed[::-1], poi_values[::-1]):.4f}\")\n",
     "print(f\"Upper limit (exp): μ = {np.interp(0.05, expected[::-1], poi_values[::-1]):.4f}\")"
    ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "Alternatively, you can also just use the [`upperlimit` API](https://pyhf.readthedocs.io/en/v0.6.0/_generated/pyhf.infer.intervals.upperlimit.html)"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "obs_limit, exp_limits, (scan, results) = pyhf.infer.intervals.upperlimit(\n",
+    "    observations, model, poi_values, level=0.05, return_results=True\n",
+    ")\n",
+    "print(f\"Upper limit (obs): μ = {obs_limit:.4f}\")\n",
+    "print(f\"Upper limit (exp): μ = {exp_limits[2]:.4f}\")"
+   ]
   }
  ],
  "metadata": {