update for FFmpeg installation on macOS via Homebrew
the latest release of FFmpeg changed the wording of the option from using "with" to "enabled"

thus the instruction would become:

```
brew install ffmpeg --enable-fdk-aac --enable-ffplay --enable-libass --enable-libvorbis --enable-libvpx --enable-rtmpdump --enable-openh264 --enable-tools
```