fix link to arXiv for asymtptotic formulae (#75)

book/HelloWorld.ipynb

@@ -632,7 +632,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "Nicely enough, `pyhf` is smart enough to let us know that, if for a model where $\\hat{\\psi} < 0$, we're not going to be able to catch it because our default bounds were bounding the parameter of interest from below at 0. For more information, see Equation 14 in [[10.7.2727]](https://arxiv.org/abs/10.7.2727)."
+    "Nicely enough, `pyhf` is smart enough to let us know that, if for a model where $\\hat{\\psi} < 0$, we're not going to be able to catch it because our default bounds were bounding the parameter of interest from below at 0. For more information, see Equation 14 in [[1007.1727]](https://arxiv.org/abs/1007.1727)."
    ]
   },
   {