Change cell type from "code" to "markdown" (#304)

tensorflow_v2/notebooks/0_Prerequisite/ml_introduction.ipynb

@@ -1,7 +1,7 @@
 {
  "cells": [
   {
-   "cell_type": "code",
+   "cell_type": "markdown",
    "execution_count": null,
    "metadata": {},
    "outputs": [],