doc: am: correct to full --no-message-id

Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: LemmingAvalanche

Documentation/git-am.adoc

@@ -91,7 +91,7 @@ OPTIONS
 
 --no-message-id::
 	Do not add the Message-ID header to the commit message.
-	`no-message-id` is useful to override `am.messageid`.
+	`--no-message-id` is useful to override `am.messageid`.
 
 -q::
 --quiet::