MAINTAINERS: change Pavel Begunkov from io_uring reviewer to maintainer

This more accurately describes Pavel's role for the project, so let's
make the change to reflect that.
Signed-off-by: Jens Axboe <axboe@kernel.dk>

MAINTAINERS

@@ -11570,7 +11570,7 @@ F:	include/linux/iosys-map.h
 
 IO_URING
 M:	Jens Axboe <axboe@kernel.dk>
-R:	Pavel Begunkov <asml.silence@gmail.com>
+M:	Pavel Begunkov <asml.silence@gmail.com>
 L:	io-uring@vger.kernel.org
 S:	Maintained
 T:	git git://git.kernel.dk/linux-block