fix error in plotting pre-treatment data

Author: drbenvincent

docs/source/notebooks/sc_pymc_brexit.ipynb

@@ -161,7 +161,7 @@
    "source": [
     "# Examine how correlated the pre-intervention time series are\n",
     "\n",
-    "pre_intervention_data = df.loc[df.index > treatment_time, :]\n",
+    "pre_intervention_data = df.loc[df.index < treatment_time, :]\n",
     "\n",
     "corr = pre_intervention_data.corr()\n",
     "\n",