Docs: corrected spelling (#1661)

Minor spelling correction

Co-authored-by: Anthonios Partheniou <partheniou@google.com>
Co-authored-by: Chalmer Lowe <chalmerlowe@google.com>
Co-authored-by: gcf-merge-on-green[bot] <60162190+gcf-merge-on-green[bot]@users.noreply.github.com>

Author: 4 people

google/oauth2/id_token.py

@@ -81,7 +81,7 @@
 def _fetch_certs(request, certs_url):
     """Fetches certificates.
 
-    Google-style cerificate endpoints return JSON in the format of
+    Google-style certificate endpoints return JSON in the format of
     ``{'key id': 'x509 certificate'}`` or a certificate array according
     to the JWK spec (see https://tools.ietf.org/html/rfc7517).