Dated TTML2 in IMSC 1.1

Hi all,

Based on the discussion at [1], I have proposed at [2] that the undated
TTML2 reference in IMSC 1.1 be changed to a dated reference to TTML2
CR2, with a plan to editorially update the reference to published TTML2
before IMSC 1.1 publication -- such a change will not be substantive since
no substantive changes can be made to TTML2 CR2.

[1] https://github.com/w3c/imsc/issues/381
[2] https://github.com/w3c/imsc/pull/412

Best,

-- Pierre