[xliff] Updated specification

FYI:

The specification document has been updated to fix an error: the pos
attribute was missing in the list of required attributes for <it>. It's
fixed. No other changes were made.

The latest specification is always at:
http://www.oasis-open.org/committees/xliff/documents/xliff-specification.htm

The list of the changes from the previous version are always at:
http://www.oasis-open.org/committees/xliff/documents/xliff-specification.htm
#AppChanges

cheers,
-yves