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

Release-As: 1.20.0

Closes #568
diff --git a/docs/reference/google.auth.compute_engine.rst b/docs/reference/google.auth.compute_engine.rst
index 38785c8..819248c 100644
--- a/docs/reference/google.auth.compute_engine.rst
+++ b/docs/reference/google.auth.compute_engine.rst
@@ -10,5 +10,6 @@
 ----------
 
 .. toctree::
+   :maxdepth: 4
 
    google.auth.compute_engine.credentials