Update example to use predefined_split properly

[eddiebergman]

From issue #1274, a user was experience issue with PredefinedSplit. It appears that our example on using it was actually using PredefinedSplit incorrectly, specifying indices instead of specifying a boolean mask.

This PR fixes the example

[codecov]

Codecov Report

Merging #1340 (ea54975) into development (b90c228) will increase coverage by 0.02%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           development    #1340      +/-   ##
===============================================
+ Coverage        87.90%   87.93%   +0.02%     
===============================================
  Files              140      140              
  Lines            10938    10938              
===============================================
+ Hits              9615     9618       +3     
+ Misses            1323     1320       -3     

Impacted Files	Coverage Δ
...learn/pipeline/components/regression/libsvm_svr.py	90.24% <0.00%> (-1.22%)	⬇️
autosklearn/ensemble_builder.py	86.23% <0.00%> (+0.40%)	⬆️
autosklearn/evaluation/abstract_evaluator.py	93.05% <0.00%> (+0.77%)	⬆️

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