pixfmt: fix wrong comment

The h264/hevc Annex E colour primaries table says that AVCOL_SPC_SMPTE170M is
similar than AVCOL_SPC_SMPTE240M. These two values are not similar than
AVCOL_SPC_BT470BG.
Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavutil/pixfmt.h

@@ -337,8 +337,8 @@ enum AVColorSpace {
     AVCOL_SPC_RESERVED    = 3,
     AVCOL_SPC_FCC         = 4,  ///< FCC Title 47 Code of Federal Regulations 73.682 (a)(20)
     AVCOL_SPC_BT470BG     = 5,  ///< also ITU-R BT601-6 625 / ITU-R BT1358 625 / ITU-R BT1700 625 PAL & SECAM / IEC 61966-2-4 xvYCC601
-    AVCOL_SPC_SMPTE170M   = 6,  ///< also ITU-R BT601-6 525 / ITU-R BT1358 525 / ITU-R BT1700 NTSC / functionally identical to above
-    AVCOL_SPC_SMPTE240M   = 7,
+    AVCOL_SPC_SMPTE170M   = 6,  ///< also ITU-R BT601-6 525 / ITU-R BT1358 525 / ITU-R BT1700 NTSC
+    AVCOL_SPC_SMPTE240M   = 7,  ///< functionally identical to above
     AVCOL_SPC_YCOCG       = 8,  ///< Used by Dirac / VC-2 and H.264 FRext, see ITU-T SG16
     AVCOL_SPC_BT2020_NCL  = 9,  ///< ITU-R BT2020 non-constant luminance system
     AVCOL_SPC_BT2020_CL   = 10, ///< ITU-R BT2020 constant luminance system