chapter 4 misspelling 'experssion' -> 'expression' (#334)

04_mnist_basics.ipynb

@@ -2109,7 +2109,7 @@
     "\n",
     "We ultimately want to write a function, `is_3`, that will decide if an arbitrary image is a 3 or a 7. It will do this by deciding which of our two \"ideal digits\" this arbitrary image is closer to. For that we need to define a notion of distance—that is, a function that calculates the distance between two images.\n",
     "\n",
-    "We can write a simple function that calculates the mean absolute error using an experssion very similar to the one we wrote in the last section:"
+    "We can write a simple function that calculates the mean absolute error using an expression very similar to the one we wrote in the last section:"
    ]
   },
   {