Fix macOS PKG_CONFIG_PATH in readme for GStreamer Binaries
I've just installed the latest 1.20.1 GStreamer .pkg on a macOS GitHub Actions CI runner and it turns out that no /Library/Frameworks/GStreamer.framework/Versions directory exists. A /Library/Frameworks/GStreamer.framework/1.0 directory is installed instead, which is more consistent with the GStreamer distributions for other OSes, and works well enough to get pkg-config working.