#11785: fix the :mod: references in email package submodule titles.

Also adds the TOC entry for headerregistry.
diff --git a/Doc/library/email.util.rst b/Doc/library/email.util.rst
index 1383104..b05c5d2 100644
--- a/Doc/library/email.util.rst
+++ b/Doc/library/email.util.rst
@@ -1,5 +1,5 @@
-:mod:`email`: Miscellaneous utilities
--------------------------------------
+:mod:`email.utils`: Miscellaneous utilities
+-------------------------------------------
 
 .. module:: email.utils
    :synopsis: Miscellaneous email package utilities.