MAINTAINERS: mark linux-doc-tw-discuss mailing list moderated

After sending a patch to [e-mail], I got
the typical response for a moderated list:

Your mail to 'linux-doc-tw-discuss' with the subject
....
Is being held until the list moderator can review it for approval.

The reason it is being held:

Post by non-member to a members-only list

Mark this mailing list moderated in MAINTAINERS.

Signed-off-by: Lukas Bulwahn <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

MAINTAINERS

···
 
 TRADITIONAL CHINESE DOCUMENTATION
 M:	Hu Haowen <[e-mail]>
-L:	[e-mail]
+L:	[e-mail] (moderated for non-subscribers)
 S:	Maintained
 W:	https://github.com/srcres258/linux-doc
 T:	git git://github.com/srcres258/linux-doc.git doc-zh-tw