website: Updated URLs to the stable branch

The master branch is to be deprecated in gem5 20.1.

Change-Id: Ib24957f690de149e08c66b697fbd8d2596b2671a
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/35436
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: Bobby R. Bruce <bbruce@ucdavis.edu>
5 files changed