[R] Update docs for ... in predict

[david-cortes]

ref #9810

R method predict has a three-dots argument (...) which needs to be in the function signature of derived predict methods, such as xgboost's.

Currently, the docs for this parameter say:

Parameters passed to \code{predict.xgb.Booster}

But the parameters aren't actually used anywhere in the function.

[trivialfis]

Thank you for the fix!