[dev] Refactor references to ap1

[aowen-uwmad]

We are changing how we incorporate the PATh facility into the coursework for this year.

The following files currently have references to ap1 and the PATh facility:

$ grep -RIl "ap1" materials/*
materials/checkpoint/part1-ex1-checkpointing.md
materials/osg/part1-ex3-hardware-diffs.md
materials/osg/part1-ex4-software-diffs.md
materials/osg/part1-ex1-login-scp.md
materials/software/part5-ex1-prepackaged.md

They should be reviewed to see if the references are appropriate given the refactor.