Update dead monkeylearn blog link (#403)

Author: AE1020

tensorflow_v2/notebooks/0_Prerequisite/ml_introduction.ipynb

@@ -13,7 +13,7 @@
     "## Machine Learning\n",
     "\n",
     "- [An Introduction to Machine Learning Theory and Its Applications: A Visual Tutorial with Examples](https://www.toptal.com/machine-learning/machine-learning-theory-an-introductory-primer)\n",
-    "- [A Gentle Guide to Machine Learning](https://blog.monkeylearn.com/a-gentle-guide-to-machine-learning/)\n",
+    "- [A Gentle Guide to Machine Learning](https://monkeylearn.com/blog/gentle-guide-to-machine-learning/)\n",
     "- [A Visual Introduction to Machine Learning](http://www.r2d3.us/visual-intro-to-machine-learning-part-1/)\n",
     "- [Introduction to Machine Learning](http://alex.smola.org/drafts/thebook.pdf)\n",
     "\n",