Include ROC AUC multiclass example in the docs

[rasbt]

Fixes #1038

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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

```diff @@ 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](https://app.codecov.io/gh/rasbt/mlxtend/pull/1041?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Sebastian+Raschka) | Coverage Δ | | |---|---|---| | [mlxtend/evaluate/holdout.py](https://app.codecov.io/gh/rasbt/mlxtend/pull/1041?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Sebastian+Raschka#diff-bWx4dGVuZC9ldmFsdWF0ZS9ob2xkb3V0LnB5) | `92.85% <100.00%> (ø)` | |

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