website: Fix building instructions

- Reordered the dependency instructions to have 20.04 dependencies
listed first.
- Fixed incorrect docker image label.

Change-Id: I5336c65770632ae8bc8230a7baf59b3ee2d6e169
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/60012
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: Bobby Bruce <bbruce@ucdavis.edu>
1 file changed