QUANTILE_AVAILABLE is set incorrectly.

[terraflops1048576]

Hi, I was looking through the source code, and I noticed this in https://github.com/ray-project/xgboost_ray/blob/master/xgboost_ray/matrix.py

try:
    from xgboost.core import QuantileDmatrix

    QUANTILE_AVAILABLE = True
except ImportError:
    QuantileDmatrix = object
    QUANTILE_AVAILABLE = False

QUANTILE_AVAILABLE is used in https://github.com/ray-project/xgboost_ray/blob/master/xgboost_ray/main.py to enable the use of QuantileDMatrix instead of DMatrix, but this is always set to be False:

import xgboost_ray.matrix
print(xgboost_ray.matrix.QUANTILE_AVAILABLE) # always prints "False"

I believe the error is one of capitalization; QuantileDmatrix should be changed to QuantileDMatrix.