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

Release-As: 1.20.0

Closes #568
diff --git a/docs/reference/google.oauth2.rst b/docs/reference/google.oauth2.rst
index 4f1df07..1ac9c73 100644
--- a/docs/reference/google.oauth2.rst
+++ b/docs/reference/google.oauth2.rst
@@ -10,6 +10,7 @@
 ----------
 
 .. toctree::
+   :maxdepth: 4
 
    google.oauth2.credentials
    google.oauth2.id_token