fix(docs): unify referenced Ubuntu versions

Documentation is inconsistent when referring to Ubuntu versioning.
Change this to a single reference that is consistent with the stated
version for TF-A tests.

The change was tested with a full build on a clean install of Ubuntu 20.04.

Signed-off-by: Boyan Karatotev <>
Change-Id: Ibb135ed938e9d92332668fa5caf274cf61b822d3
2 files changed