fix a typo

examples/univariate_volatility_modeling.ipynb

@@ -401,7 +401,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "Volatility processes can be added a a mean model using the `volatility` property.  This example adds an ARCH(5) process to model volatility. The arguments `iter` and `disp` are used in `fit()` to suppress estimation output."
+    "Volatility processes can be added to a mean model using the `volatility` property.  This example adds an ARCH(5) process to model volatility. The arguments `iter` and `disp` are used in `fit()` to suppress estimation output."
    ]
   },
   {