Update documentation

Author: AllenDowney

_sources/preface.md

@@ -121,7 +121,7 @@ To run the notebooks on your computer, there are three steps:
 
 3.  Run Jupyter and open the notebooks.
 
-To get the notebooks, download [this Zip archive](http://modsimpy.com/zip). You will need a program like
+To get the notebooks, download [this Zip archive](https://github.com/AllenDowney/ModSimPy/raw/refs/heads/master/ModSimPyNotebooks.zip). You will need a program like
 WinZip or gzip to unpack the Zip file. Make a note of the location of
 the files you download.
 

preface.html

@@ -463,7 +463,7 @@ <h1>Preface<a class="headerlink" href="#preface" title="Link to this heading">#<
 <li><p>Install Python, Jupyter, and some additional libraries.</p></li>
 <li><p>Run Jupyter and open the notebooks.</p></li>
 </ol>
-<p>To get the notebooks, download <a class="reference external" href="http://modsimpy.com/zip">this Zip archive</a>. You will need a program like
+<p>To get the notebooks, download <a class="reference external" href="https://github.com/AllenDowney/ModSimPy/raw/refs/heads/master/ModSimPyNotebooks.zip">this Zip archive</a>. You will need a program like
 WinZip or gzip to unpack the Zip file. Make a note of the location of
 the files you download.</p>
 <p>The next two sections provide details for the other steps.