manual.txt: Follow README.md and remove Jon

Following 0ee1d479b2 ("Remove Jon Loeliger from maintainers list"),
make the "Submitting Patches" section of the manual.txt consistent with
the README by requesting patches to only be sent to David.

Cc: Jon Loeliger <loeliger@gmail.com>
Signed-off-by: Pierre-Clément Tosi <ptosi@google.com>
Message-Id: <20220729131019.806164-1-ptosi@google.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

Documentation/manual.txt

@@ -43,9 +43,8 @@ The gitweb interface for the upstream repository is:
 
 1.1) Submitting Patches
 
-Patches should be sent to the maintainers:
+Patches should be sent to the maintainer:
 	David Gibson <david@gibson.dropbear.id.au>
-	Jon Loeliger <loeliger@gmail.com>
 and CCed to <devicetree-compiler@vger.kernel.org>.
 
 2) Description