[css-tables] Clarify the algo of step 3.11

[gregwhitworth]

I was reviewing a specific test case and was going through the algo, ended up at 3.11 and found it kind of confusing. The spec text reads as though it's defining something when it should read as a regular math algo for positioning the boxes, similar to the over-arching layout algo does: https://drafts.csswg.org/css-tables-3/#table-layout-algorithm