FUML15 — Base Semantics PSL version

Base Semantics should declare the PSL version used to define it.

Update footnote 2 in subclause 10.1

There has only been one internationally standardized version of PSL so far. However, just in case there may eventually be an update, the reference in footnote 2 in subclause 10.1 can be updated to specifically cite "ISO 18629-1:2004", which is the overview part of the standard as of 2004 (which is still the current version). In addition, the link http://www.nist.gov/psl now seems to redirect to a general NIST Engineering Laboratory page, not a page on PSL, so that should also be removed from the footnote.