Minor tweaks to original definition of concurrency

Thank you for sharing this thesis, I have really enjoyed reading it!

Minor tweaks to original definition of concurrency, as it may be mis-leading to suggest that two or more activities *have* to be executed at the same time, a system can be concurrent but not execute exactly at the same time, for instance time slicing on multi-tasking single core machine that's inferred to later in the thesis. See http://docs.oracle.com/cd/E19455-01/806-5257/6je9h032b/index.html and http://stackoverflow.com/a/1050257/192040