Update book1.html for 2nd printing of book1

Author: murphyk

book1.html

@@ -21,7 +21,7 @@ <h2>Key links</h2>
   <li> <a href="preface1.pdf">Preface</a>
 
   <li> <a href="https://github.com/probml/pml-book/releases/latest/download/book1.pdf">
-    Draft pdf file</a>, 2022-08-08.  CC-BY-NC-ND license. (Please cite the official reference below.)
+    Draft pdf file</a>, 2023-04-17 (2nd printing).  CC-BY-NC-ND license. (Please cite the official reference below.)
 
 	<li> <a href="https://github.com/probml/pml-book/issues">Report issues here</a>