update build guide

The build guide is here:

https://git.purrdata.net/jwilkes/purr-data#build-guide

The first sentence says this:

The instructions below talk about running the tar_em_up.sh build script, which is still the recommended way to build Purr Data right now.

It should be changed to this:

The instructions for Windows and OSX below talk about running the tar_em_up.sh build script, which is still the recommended way to build Purr Data right now on those platforms.