[Spec change] Fix definition of RET for vec operators

Please review whether https://github.com/KhronosGroup/SYCL-Docs/pull/666 by @gmlueck requires any changes to the CTS.

**If changes are required**: Open a new PR addressing the changes and link it to this issue.
**If no changes are required**: Close this issue and proceed with the spec PR.

Start by reviewing the linked SYCL-Docs pull request 666 and compare its definition of RET for vector operators with the current CTS coverage. Determine whether the CTS needs changes; if so, open a new pull request and link it here, otherwise close this issue and proceed with the specification pull request.

Written by the indexing model from the issue text.