Documentation for VMAF usage refers to unsupported pkl Model instead of json

[Araeos]

The README.md has an entry (see quote below) for calculating the VMAF results with a user supplied model and in the example names a pkl file, but at least with my version of ffmpeg ( 2021-12-30-git-12f21849e5-full_build) pkl models are explicitly not supported anymore and json is recommended.

--vmaf-path             Custom path to libvmaf models.
                        example: --vmaf-path "vmaf_v0.6.1.pkl"
                        Recommended to place both files in encoding folder
                        (`vmaf_v0.6.1.pkl` and `vmaf_v0.6.1.pkl.model`)
                        (Required if VMAF calculation doesn't work by default)

Just a simple update would be appreciated, but I am not confident in creating a pull request. Thanks for developing and providing Av1an.