Consistent spelling error in documentation

[rien333]

When searching for class imbalance methods that are able to deal with multi-class problems in the documentation, I had a hard time finding and refinding algorithms. Turns out that sometimes "multi" is written as "mutli" on multiple pages, e.g. the notes section of SMOTE. (imagine all the people on google not being able to find this package for their multiclass problems!)

[glemaitre]

Good catch. PR welcomed.

[gnsiva]

I can do this one 👍