bpo-42770: Fix a typo in the email.headerregistry docs (GH-23982)

Automerge-Triggered-By: GH:zware

Doc/library/email.headerregistry.rst

@@ -289,7 +289,7 @@
     A :class:`ParameterizedMIMEHeader` class that handles the
     :mailheader:`Content-Disposition` header.
 
-    .. attribute:: content-disposition
+    .. attribute:: content_disposition
 
        ``inline`` and ``attachment`` are the only valid values in common use.