[docs][python] fix LGBMRanker docstring (#4306)
[docs][python] fix LGBMRanker docstring (#4306)


henry0312/LightGBM,henry0312/LightGBM,henry0312/LightGBM,microsoft/LightGBM,microsoft/LightGBM,henry0312/LightGBM,microsoft/LightGBM,henry0312/LightGBM,microsoft/LightGBM,microsoft/LightGBM