Fix name of ocv package

[rslota]

Problem

> $ darwin dataset pull longvideosnoframes/videos-no-frames-extraction:2 --video-frames                                 
Error: Missing Dependency: OpenCV required for Video Extraction. Install with `pip install darwin[ocv]`
                                                                                                                         
> $ pip install darwin\[ocv\]                                                                                           
ERROR: Could not find a version that satisfies the requirement darwin[ocv] (from versions: none)
ERROR: No matching distribution found for darwin[ocv]

Solution

Rename darwin[ocv] -> darwin-py[ocv]