update docs per #33

[codecov-commenter]

Codecov Report

Merging #34 (9c14709) into dev (d908f77) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev      #34   +/-   ##
=======================================
  Coverage   86.88%   86.88%           
=======================================
  Files           1        1           
  Lines         122      122           
=======================================
  Hits          106      106           
  Misses         16       16           

Impacted Files	Coverage Δ
src/MLJDecisionTreeInterface.jl	86.88% <ø> (ø)

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[john-waczak]

updating documentation to remove :n_subfeatures=-1 as option for DecisionTreeRegressor and DecisionTreeClassifier per issue #33