Out-dated installation folder for Mac OS in README.md

[glaza]

Hi Marty,

First of all, thank you for this plug-in, it is fantastic!

Just wanted to let you know that Inkscape installs differently on High Sierra and Mojave.
Official Instructions here

The folder we should copy the src/* is: /opt/local/share/inkscape/extensions

Thanks,

George

[Marty McGuire]

Thanks! I've updated the README.