Make a nice screenshot for the homepage.

Author: pauleveritt

src/psc/gallery/examples/altair/screenshot.png

@@ -16,7 +16,7 @@
                         <div class="card">
                             <div class="card-image">
                                 <figure class="image is-square">
-                                    <img alt="placeholder" src="{{ root_path }}/gallery/examples/hello_world/screenshot.png">
+                                    <img alt="placeholder" src="{{ root_path }}/gallery/examples/altair/screenshot.png">
                                 </figure>
                             </div>
                             <div class="card-content">