fix typo: You don't to -> You don't have to

Author: Yi Xu

notebooks/02_NumPy.ipynb

@@ -438,8 +438,8 @@
       },
       "cell_type": "markdown",
       "source": [
-        "You don't to memorize anything here and we will be taking a closer look at NumPy in the later lessons. If you are curious about more checkout the [NumPy reference manual](https://docs.scipy.org/doc/numpy-1.15.1/reference/)."
+        "You don't have to memorize anything here and we will be taking a closer look at NumPy in the later lessons. If you are curious about more checkout the [NumPy reference manual](https://docs.scipy.org/doc/numpy-1.15.1/reference/)."
       ]
     }
   ]
-}
+}