Include ROC AUC multiclass example in the docs

[rasbt]

Fixes #1038

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[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (a60f127) 77.27% compared to head (63f080c) 77.27%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1041   +/-   ##
=======================================
  Coverage   77.27%   77.27%           
=======================================
  Files         200      200           
  Lines       11307    11307           
  Branches     1484     1484           
=======================================
  Hits         8737     8737           
  Misses       2351     2351           
  Partials      219      219           

Impacted Files	Coverage Δ
mlxtend/evaluate/holdout.py	92.85% <100.00%> (ø)

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[rasbt]

ok