auto_examples: fix index.rst for CI workflow

Problem: sphinx-gallery has rolled some updates
in the last few months. As a working theory, one of
these updates might have required a new div in the
auto-generated gallery we use for displaying examples.

Update that gallery's index.rst to get the CI working
again.

auto_examples/index.rst

@@ -22,6 +22,7 @@ of using the Flux Python API.
 
     <div class="sphx-glr-thumbnails">
 
+.. thumbnail-parent-div-open
 
 .. raw:: html
 
@@ -40,6 +41,8 @@ of using the Flux Python API.
     </div>
 
 
+.. thumbnail-parent-div-close
+
 .. raw:: html
 
     </div>