media: dmx.h documentation: fix a warning

/devel/v4l/patchwork/Documentation/output/dmx.h.rst:6: WARNING: undefined label: dmx_dqbuf (if the link has no caption the label must precede a section header)

This is defined together with DMX_QBUF.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/media/dmx.h.rst.exceptions

@@ -54,3 +54,5 @@ ignore symbol DMX_OUT_DECODER
 ignore symbol DMX_OUT_TAP
 ignore symbol DMX_OUT_TS_TAP
 ignore symbol DMX_OUT_TSDEMUX_TAP
+
+replace ioctl DMX_DQBUF dmx_qbuf