fix link from mrdbourke#536

Author: mrdbourke

README.md

@@ -157,7 +157,7 @@ You can find solutions to the exercises in [`extras/solutions/`](https://github.
 ### 📖 02. Neural network classification with TensorFlow Extra-curriculum
 
 * Watch 3Blue1Brown's neural networks video 2: [*Gradient descent, how neural networks learn*](https://www.youtube.com/watch?v=IHZwWFHWa-w). After you're done, write 100 words about what you've learned.
-  * If you haven't already, watch video 1: [*But what is a Neural Network?*](https://www.youtube.com/watch?v=7sB052Pz0sQ&ab_channel=AlexanderAmini). Note the activation function they talk about at the end.
+  * If you haven't already, watch video 1: [*But what is a Neural Network?*](https://www.youtube.com/watch?v=aircAruvnKk). Note the activation function they talk about at the end.
 * Watch [MIT's introduction to deep learning lecture 1](https://youtu.be/njKP3FqW3Sk) (if you haven't already) to get an idea of the concepts behind using linear and non-linear functions.
 * Spend 1-hour reading [Michael Nielsen's Neural Networks and Deep Learning book](http://neuralnetworksanddeeplearning.com/index.html).
 * Read the [ML-Glossary documentation on activation functions](https://ml-cheatsheet.readthedocs.io/en/latest/activation_functions.html). Which one is your favourite?