Tweak to section 4

Author: Christopher Fonnesbeck

4. Statistical Data Modeling.ipynb

@@ -1145,8 +1145,8 @@
      "input": [
       "from statsmodels.formula.api import ols as OLS\n",
       "\n",
-      "data = pd.DataFrame(dict(x=x, x2=x**2, y=y))\n",
-      "cubic_fit = OLS('y ~ x + x2', data).fit()\n",
+      "data = pd.DataFrame(dict(x=x, y=y))\n",
+      "cubic_fit = OLS('y ~ x + I(x**2)', data).fit()\n",
       "\n",
       "cubic_fit.summary()"
      ],