Updates link to book

Author: mwray

.gitignore

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       <h2>Textbook</h2>
-      <p>Recommended Reading:<a href="" target="_blank">Simon J.D (2023). Prince. Understanding Deep Learning, MIT Press</a><br/></p>
+      <p>Recommended Reading:<a href="https://udlbook.github.io/udlbook/" target="_blank">Simon J.D (2023). Prince. Understanding Deep Learning, MIT Press</a><br/></p>
 
       You can also check out, written pre transformers, the older course book: <a href="http://www.deeplearningbook.org/" target="_blank">Goodfellow et al (2016). Deep Learning. MIT Press</a>
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