media: videobuf2-dma-contig.c: remove spurious 'b' in message

Remove a spurious 'b' in the "contiguous chunk is too small  %lu/%lu b"
message.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>