Perceptual difference between `scale` and `zscale` after the OCIO filter

When following the `FfmpegOcio.md` guidance to convert from RGB (OCIO filter output) to YCbCr using the `scale` filter, I noticed a perceptible difference from the EXR source. Swapping `scale` for `zscale` resolved it and much closer to the source in my testing.

**Example:**

```
ffmpeg -y -framerate 24 -start_number -i SOURCEFRAMES.%05d.exr \
-c:v prores_ks -profile:v 1 -pix_fmt yuv422p10le \
-vf "ocio=input=ACEScg:output=ACEScct:format=rgb48,zscale=matrix=709:primaries=709:primariesin=709:transfer=unspecified:transferin=unspecified:rangein=pc:range=limited" \
-movflags +faststart+write_colr -vendor apl0 OUTPUTFILE.mov
```

I don't have CLA clearance setup yet, so sharing my branch here instead: [`ffmpeg-ocio-zscale-command`](https://github.com/johnjoumaa/EncodingGuidelines/commit/0ca5457c3ad7a989aa77c7b59ffed3068f458940) on my fork, which adds a `zscale` note + alternate examples to `FfmpegOcio.md` alongside the existing `scale`-based ones. Happy to edit, have someone pick it up, or I can open the PR myself once CLA is sorted.

Let me know what your prefer,

Cheers!

从 FfmpegOcio.md 开始，将其中现有的基于 scale 的指导与链接的 ffmpeg-ocio-zscale-command commit 进行比较。确认 zscale 注释和替代示例准确反映所报告的差异，并在纳入这些补充内容且示例与现有指导保持一致后，认为文档已完成。

由索引模型根据 Issue 内容生成。