[media] pixfmt-007.rst: fix copy-and-paste error in SMPTE-240M doc

Instead of referring to Y', Cb and Cr, it referred to Yc', Cbc and
Crc, which were accidentally copied from the BT.2020 constant luminance
text.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/media/uapi/v4l/pixfmt-007.rst

@@ -742,7 +742,7 @@ the following ``V4L2_YCBCR_ENC_SMPTE240M`` encoding:
 
     Cr = 0.5R' - 0.4451G' - 0.0549B'
 
-Yc' is clamped to the range [0…1] and Cbc and Crc are clamped to the
+Y' is clamped to the range [0…1] and Cb and Cr are clamped to the
 range [-0.5…0.5]. The Y'CbCr quantization is limited range.