[css-regions-1] Fix inconsistency aliases

[alkorlos]

Fix bikeshed errors.

The command:

bikeshed spec css-regions-1\Overview.bs

Throws errors:

LINE ~627: The biblio refs [[CSS21]] and [[CSS2]] are both aliases of the same base reference [[CSS21]]. Please choose one name and use it consistently.
LINE ~1631: The biblio refs [[CSS21]] and [[CSS2]] are both aliases of the same base reference [[CSS21]]. Please choose one name and use it consistently.

A search through the repositories shows that the alias [[!CSS2]] is used three times more often than [[!CSS21]], so I replaced all aliases with [[!CSS2]].

[w3cbot]

frivoal marked as non substantive for IPR from ash-nazg.