Update book2.html

Change version number to 2023-04-01.

Author: murphyk

book2.html

@@ -23,7 +23,7 @@ <h2>Key links</h2>
 <li> <a href="https://github.com/probml/pml2-book/blob/main/preface2-2023-01-02.pdf">Preface</a>
 
   <li> <a href="https://github.com/probml/pml2-book/releases/latest/download/book2.pdf">
-    Draft pdf of the main book</a>, 2023-01-19.  CC-BY-NC-ND license. (Please cite the official reference below.)
+    Draft pdf of the main book</a>, 2023-04-01.  CC-BY-NC-ND license. (Please cite the official reference below.)
 
 <li> <a href="https://github.com/probml/pml-book/blob/main/supp2.md">Supplementary material</a>