doc/filters: remove outdated showwavespic example

Use "colors" option instead.
Signed-off-by: Lou Logan <lou@lrcd.com>

doc/filters.texi

@@ -16705,14 +16705,6 @@ in a 1024x800 picture using @command{ffmpeg}:
 @example
 ffmpeg -i audio.flac -lavfi showwavespic=split_channels=1:s=1024x800 waveform.png
 @end example
-
-@item
-Colorize the waveform with colorchannelmixer. This example will make
-the waveform a green color approximately RGB(66,217,150). Additional
-channels will be shades of this color.
-@example
-ffmpeg -i audio.mp3 -filter_complex "showwavespic,colorchannelmixer=rr=66/255:gg=217/255:bb=150/255" waveform.png
-@end example
 @end itemize
 
 @section spectrumsynth