[3.14] Docs: Fix typo in `email.headerregistry.rst` (GH-140965) (#141020)

Docs: Fix typo in `email.headerregistry.rst` (GH-140965)

Fix missing 'Header' suffix on header class name in `email.headerregistry.rst`
(cherry picked from commit 97d8dda980fcddf88b782be343118257f483a864)

Co-authored-by: Guo Ci <zguoci@gmail.com>