Address `Subsets` to-do about clearer members names

[atifaziz]

This PR addresses the following to-do comment in Subsets:

https://github.com/morelinq/MoreLINQ/blob/80ab81c9e06af0409e621ddeef7f83e4f4ebf0b9/MoreLinq/Subsets.cs#L139

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (80ab81c) 92.51% compared to head (a0f72ab) 92.50%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1048 +/- ## ========================================== - Coverage 92.51% 92.50% -0.01% ========================================== Files 112 112 Lines 3405 3404 -1 Branches 1056 1056 ========================================== - Hits 3150 3149 -1 Misses 189 189 Partials 66 66 ```

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