docs/manual: fix description of how to reply to <message-id>

Patch v1 not set to 'superseded' when replying to <message-id> with v2.

I'm using git version 2.46.0. The manual says the following is the
correct way to use the '--in-reply-to' option:

git send-email --in-reply-to=<message-id>

Signed-off-by: Roy Kollen Svendsen <roykollensvendsen@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

docs/manual/contribute.adoc

@@ -517,7 +517,7 @@ address you register in
 https://patchwork.ozlabs.org/project/buildroot/list/[patchwork] should
 match the one you use for sending patches to the mailing list.
 
-You can also add the +--in-reply-to <message-id>+ option when
+You can also add the +--in-reply-to=<message-id>+ option when
 submitting a patch to the mailing list. The id of the mail to reply to
 can be found under the "Message Id" tag on
 https://patchwork.ozlabs.org/project/buildroot/list/[patchwork]. The