docs(python): fix intersphinx link for google-auth (#860)

Author: busunkim96

synthtool/gcp/templates/python_library/docs/conf.py.j2

@@ -346,7 +346,7 @@ texinfo_documents = [
 # Example configuration for intersphinx: refer to the Python standard library.
 intersphinx_mapping = {
     "python": ("https://python.readthedocs.org/en/latest/", None),
-    "google-auth": ("https://googleapis.dev/python/google-auth/latest/index.html", None),
+    "google-auth": ("https://googleapis.dev/python/google-auth/latest/", None),
     "google.api_core": (
         "https://googleapis.dev/python/google-api-core/latest/",
         None,