website: Made changes to learning_gem5 tutorial

Changed variable names in part1_3 to match the files linked
in that tutorial, and changed pointers + references in part2_4
to be in line with files linked in that tutorial.

Change-Id: If59405bd6d2de3710b8eda785b3e28090b20ae11
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/47840
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: Bobby R. Bruce <bbruce@ucdavis.edu>