remove unneeded references to "master"

[chalmerlowe]

We did an initial scrub to remove old references to the master git branch and replace it with main. Unfortunately, we did not catch all the references, so we need to do another scrub.

WARNING: do not do a simple global find and replace. There is a sentence OR two where we explain that:

• "master" was commonly used but has been broadly phased out as inappropriate
• "main" is a widely used replacement

We do not want to lose that context and messaging.

[Travis-Magaluk]

This should be resolved now. See the image of the approved pull request below.