[tei-council] rendering of @validUntil in the Guidelines

[Sebastian Rahtz]

see 

http://bits.nsms.ox.ac.uk:8080/jenkins/job/TEIP5/lastSuccessfulBuild/artifact/release/doc/tei-p5-doc/en/html/ref-join.html
http://bits.nsms.ox.ac.uk:8080/jenkins/job/TEIP5/lastSuccessfulBuild/artifact/release/doc/tei-p5-doc/en/html/ref-relationGrp.html

one bit more to do.

shout now if you want a different form of words.
--
Sebastian Rahtz      
Director (Research) of Academic IT
University of Oxford IT Services
13 Banbury Road, Oxford OX2 6NN. Phone +44 1865 283431