This edits the documentation regarding the usage of the --isa tag, as this has fallen out of date in regards to the new 'ALL' isa. Change-Id: I3b672ac2c03dd109bba458db688af05ed4135a91 Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/65651 Tested-by: kokoro <noreply+kokoro@google.com> Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu> Maintainer: Bobby Bruce <bbruce@ucdavis.edu>