docs: fix note rendering for service_account_email (#385)

diff --git a/google/auth/compute_engine/credentials.py b/google/auth/compute_engine/credentials.py
index eeb92f5..0abfc05 100644
--- a/google/auth/compute_engine/credentials.py
+++ b/google/auth/compute_engine/credentials.py
@@ -105,7 +105,7 @@
     def service_account_email(self):
         """The service account email.
 
-        .. note: This is not guaranteed to be set until :meth`refresh` has been
+        .. note:: This is not guaranteed to be set until :meth:`refresh` has been
             called.
         """
         return self._service_account_email