Fixed a typo in Preliminaries (prereqs)

01 - Preliminaries.ipynb

@@ -102,7 +102,7 @@
     "(and for the *second part* of the tutorial):\n",
     "\n",
     "- `scikit-learn` version 0.12 or later: http://scikit-learn.org\n",
-    "- `networkx` version 1.9.1 or later: https://networkx.github.io\n",
+    "- `numexpr` version 2.4.1 or later: https://github.com/pydata/numexpr\n",
     "\n",
     "The easiest way to get these is to use an all-in-one installer such as [Anaconda](http://www.continuum.io/downloads) from Continuum. These are available for multiple architectures."
    ]