Deprecated reference at the end of the NLP crown jewel Encoder decoder lab (`rnn_encoder_decoder.ipynb`)

In my opinion this is the crown jewel lab of the whole NLP course of the Machine Learning Engineer learning path. The CSB (Cloud Skills Boost) lab is titled simply "Encoder decoder" (https://www.cloudskillsboost.google/course_sessions/2920308/labs/363239), while the notebook title tells away more "Simple RNN Encode-Decoder for Translation" https://github.com/GoogleCloudPlatform/training-data-analyst/blob/master/courses/machine_learning/deepdive2/text_classification/labs/rnn_encoder_decoder.ipynb

This is no biggie but the reference at the end is 404. I believe the correct link now is https://github.com/keras-team/keras-io/blob/master/examples/nlp/lstm_seq2seq.py which is also authored by Francois Chollet.

As usual (when it's not a CSB correction), I'm crafting a PR.