README: Change the tested Ubuntu version

The README.md contains information on how to build the documentation locally.
Change the version of Ubuntu at which this build has been tested to a more
updated one and to be consistent with the rest of the documentation.

Issue-Id: SCM-5314
Signed-off-by: Tomás González <tomasagustin.gonzalezorlando@arm.com>
Change-Id: I0d5a4d02a0ddccb9ba7dbe366448e7fbc58ae1f7