MAINTAINERS: Fix F: in "virtio-snd" and "virtio-gpu"

We neglected to update MAINTAINERS when we moved docs/system/devices/virtio-snd.rst and docs/system/devices/virtio-gpu.rst to docs/system/devices/virtio/. Update it now. Fixes: 487ce6ce99 (docs/system: split VirtIO devices from the rest, 2025-10-16) Cc: Gerd Hoffmann <kraxel@redhat.com> Cc: Manos Pitsidianakis <manos.pitsidianakis@linaro.org> Cc: Alex Bennée <alex.bennee@linaro.org> Cc: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp> Cc: Dmitry Osipenko <dmitry.osipenko@collabora.com> Signed-off-by: Markus Armbruster <armbru@redhat.com> Reviewed-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org> Reviewed-by: Alex Bennée <alex.bennee@linaro.org> Reviewed-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp> Reviewed-by: Dmitry Osipenko <dmitry.osipenko@collabora.com> Message-ID: <20260603064032.3733394-25-armbru@redhat.com>