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 <boyan.karatotev@arm.com>
Change-Id: Ibb135ed938e9d92332668fa5caf274cf61b822d3
diff --git a/docs/getting_started/prerequisites.rst b/docs/getting_started/prerequisites.rst
index 62ef826..d8d3b7a 100644
--- a/docs/getting_started/prerequisites.rst
+++ b/docs/getting_started/prerequisites.rst
@@ -14,7 +14,7 @@
 |TF-A| can be built using either a Linux or a Windows machine as the build host.
 
 A relatively recent Linux distribution is recommended for building |TF-A|. We
-have performed tests using Ubuntu 16.04 LTS (64-bit) but other distributions
+have performed tests using Ubuntu 20.04 LTS (64-bit) but other distributions
 should also work fine as a base, provided that the necessary tools and libraries
 can be installed.