Rename one variable

To enhance code clarity and prevent potential confusion with the
'r' variable assigned to reduction later in the code, this pull
request renames it to 'reductionScale' when we use the same name
in the reduction() function.

Using distinct variable names for separate functions improves code
readability and maintainability.

closes https://github.com/official-stockfish/Stockfish/pull/4765

No functional change