feat(packaging): add support for Python 3.8 (#569)

Release-As: 1.20.0

Closes #568
diff --git a/docs/reference/google.rst b/docs/reference/google.rst
index 4b1e085..f122ca1 100644
--- a/docs/reference/google.rst
+++ b/docs/reference/google.rst
@@ -10,6 +10,7 @@
 -----------
 
 .. toctree::
+   :maxdepth: 4
 
    google.auth
    google.oauth2