[ML][MINOR] Invalid MulticlassClassification reference in ml-guide

In [ml-guide](https://spark.apache.org/docs/latest/ml-guide.html#example-model-selection-via-cross-validation), there is invalid reference to `MulticlassClassificationEvaluator` apidoc.

https://spark.apache.org/docs/latest/api/scala/index.html#org.apache.spark.ml.evaluation.MultiClassClassificationEvaluator

Author: Lewuathe <lewuathe@me.com>

Closes #10996 from Lewuathe/fix-typo-in-ml-guide.