DOC: Fix thumbnail title for sphinx gallery

Sphinx Gallery 0.11.0 left RST formatting characters in the thumbnail
titles. This was fixed in version 0.12.2 for most cases by
github.com/sphinx-gallery/sphinx-gallery/pull/1005, which doesn't
cover default roles. So to restore the previous behavior we need
to explicitely a role.

Author: StefRe

galleries/examples/axes_grid1/demo_colorbar_with_axes_divider.py

@@ -1,7 +1,7 @@
 """
-============================
-Colorbar with `.AxesDivider`
-============================
+==================================
+Colorbar with :obj:`~.AxesDivider`
+==================================
 
 The `.axes_divider.make_axes_locatable` function takes an existing axes, adds
 it to a new `.AxesDivider` and returns the `.AxesDivider`.  The `.append_axes`