fix link

index.html

@@ -69,7 +69,7 @@ <h2><span class="fa fa-book"></span> Introduction to Computational Astrophysical
 	      <a href="https://github.com/Open-Astrophysics-Bookshelf/numerical_exercises"> github repo</a></span>
 	    &nbsp;
 	    <span class="fa fa-file-pdf-o">
-	      <a href="http://bender.astro.sunysb.edu/hydro_by_example/CompHydroTutorial.pdf"> PDF version</a></span>
+	      <a href="https://open-astrophysics-bookshelf.github.io/numerical_exercises/CompHydroTutorial.pdf"> PDF version</a></span>
 	  <p>An introduction to the grid-based methods commonly used
 	    for astrophysical simulation, starting with linear
 	    advection and developing the methods for the compressible