[tests][dask] fix argument names in custom eval function in Dask test (#4833)
[tests][dask] fix argument names in custom eval function in Dask test (#4833)

* fix argument types in custom eval function for Dask estimators

* revert changes to docstrings

* fix argument names in Dask test
henry0312/LightGBM,henry0312/LightGBM,microsoft/LightGBM,henry0312/LightGBM,microsoft/LightGBM,henry0312/LightGBM,henry0312/LightGBM,microsoft/LightGBM,microsoft/LightGBM,microsoft/LightGBM