Update seasonality,_holiday_effects,_and_regressors.ipynb (#633)

just a typo fix

notebooks/seasonality,_holiday_effects,_and_regressors.ipynb

@@ -123,7 +123,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "Above we have include the superbowl days as both playoff games and superbowl games. This means that the superbowl effect will be an additional additive bonus on top of the playoff effect.\n",
+    "Above we have included the superbowl days as both playoff games and superbowl games. This means that the superbowl effect will be an additional additive bonus on top of the playoff effect.\n",
     "\n",
     "Once the table is created, holiday effects are included in the forecast by passing them in with the `holidays` argument. Here we do it with the Peyton Manning data from the Quickstart:"
    ]