[SYCL][Docs] Invert group numbering for ballot groups

[steffenlarsen]

The current extension for non-uniform groups state that ballot groups are numbered 0 if the predicate was true and 1 if the predicate was false. However, the implementation has the inverse numbering. This commit changes the extension to make the numbering consistent with the implementation, in turn making the group numbering directly convertible to a boolean value equal to the corresponding predicate.

[Pennycook]

While we're here, how would you feel about updating this line too, in the definition of get_local_range():

This will always return a range of 2, as there will always be two groups; one representing the group of work-items where the predicate was true and another representing the group of work-items where the predicate was false.

To say:

This will always return a range of 2, as there will always be two groups; one representing the group of work-items where the predicate was true and another representing the group of work-items where the predicate was false. It is possible for one of these groups to contain no work-items, if the predicate has the same value on all work-items.