[3.7] bpo-35035: Rename email.utils documentation to email.utils.rst (GH-10023)

I'll watch for 404 on the old URL and will setup an HTTP redirection if needed.
(cherry picked from commit 361e8683e7340c600b22f4a514b81448ccec66dc)

Co-authored-by: Zhiming Wang <github@zmwang.pw>
diff --git a/Doc/library/email.rst b/Doc/library/email.rst
index 07d455b..fae99cf 100644
--- a/Doc/library/email.rst
+++ b/Doc/library/email.rst
@@ -126,7 +126,7 @@
    email.header.rst
    email.charset.rst
    email.encoders.rst
-   email.util.rst
+   email.utils.rst
    email.iterators.rst