demo's doc: Update README.MD to fix typo

[keni-chi]

I noticed "Benckmark" was used at the demo/kaggle-higgs/README.MD instead of "Benchmark".
If that was intentional then ignore this change.

[hcho3]

@trivialfis @RAMitchell I'm seeing this error lately in unit test:

[ RUN      ] GpuHist.ApplySplit
unknown file: Failure
C++ exception with description "vector::_M_range_check" thrown in the test body.

EDIT. Seeing the same error in #4155
Any idea what's going on?

[trivialfis]

That's weird. Seems not deterministic. Will try looking into it.

[codecov-io]

Codecov Report

Merging #4156 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4156      +/-   ##
==========================================
+ Coverage   63.37%   63.37%   +<.01%     
==========================================
  Files         131      131              
  Lines       11982    11983       +1     
==========================================
+ Hits         7593     7594       +1     
  Misses       4389     4389

Impacted Files	Coverage Δ
src/tree/split_evaluator.cc	49.79% <0%> (ø)	⬆️
src/gbm/gbtree_model.h	44.82% <0%> (ø)	⬆️
src/tree/split_evaluator.h	100% <0%> (ø)	⬆️
include/xgboost/predictor.h	100% <0%> (ø)	⬆️
src/data/sparse_page_dmatrix.h	100% <0%> (ø)	⬆️
src/tree/updater_quantile_hist.h	18.05% <0%> (ø)	⬆️
src/data/simple_dmatrix.h	100% <0%> (ø)	⬆️
include/xgboost/learner.h	100% <0%> (ø)	⬆️
src/common/transform.h	88.88% <0%> (ø)	⬆️
src/common/host_device_vector.h	75% <0%> (ø)	⬆️
... and 1 more

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[trivialfis]

@keni-chi Thanks!