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

Also adds the TOC entry for headerregistry.
diff --git a/Doc/library/email.rst b/Doc/library/email.rst
index fc206f4..ef5354f 100644
--- a/Doc/library/email.rst
+++ b/Doc/library/email.rst
@@ -52,6 +52,7 @@
    email.parser.rst
    email.generator.rst
    email.policy.rst
+   email.headerregistry.rst
    email.mime.rst
    email.header.rst
    email.charset.rst