Order code terms to match order of equation terms (ProjectPythia#477)

Reverse the order of the 2nd and 3rd terms in the code corresponding to
the finite difference estimate of the 2nd derivative of a function to
match the order of the terms in the equation.

core/numpy/numpy-broadcasting.ipynb

@@ -587,7 +587,7 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "2 * a[1:-1] - a[:-2] - a[2:]"
+    "2 * a[1:-1] - a[2:] - a[:-2]"
    ]
   },
   {