doc: Update license description in README to be consistent with documentation

Also tweak the description in both the README and the rendered documentation to
remove redundant wording, and fix a typo from the introduction.

Issue-Id: SCM-4394
Signed-off-by: Richard Neill <richard.neill@arm.com>
Change-Id: Idbb9e79f3b9c29b72f4ee598b19178c1fdc99197